Re: [PATCH] i3c: master: dw: Clamp GETMRL/GETMWL to controller FIFO limits

["Patil, Shubham Sanjay" <shubpati@amd.com>]

On 9/11/2026 12:04 AM, Frank Li wrote:
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> 
> 
> On Tue, Sep 08, 2026 at 03:57:24PM +0530, Shubham Patil wrote:
>> The DW master rejects private SDR transfers larger than
>> caps.datafifodepth with -EOPNOTSUPP. Targets often report MRL/MWL
>> values larger than that FIFO, so the core stores limits the controller
>> cannot meet.
>>
>> After a successful GETMRL/GETMWL, issue Direct SETMRL/SETMWL to the
>> same target with lengths capped to the data FIFO (in bytes), then
>> rewrite the GET payload so the core keeps the same values. Only update
>> the GET buffer once SET is acked, so a failed SET does not leave the
>> core and the target disagreeing.
> 
> I think i3c device driver should know these information choose
> min value dring each xfer. even though you set devcie's MRL/MXL, device
> driver still issue a longer transfer.
> 
> Frank

Understood - I will drop the SETMRL/SETMWL and stop rewriting the GET
payload, and instead expose the controller limit so the min is taken
per transfer. Two questions on how you want that done:
1) Where should the min be taken?
    a) In the core: the controller driver sets max_read_len /
       max_write_len / max_ibi_len in struct i3c_master_controller, and
       the core caps i3c_device_info to min(target, controller) after
       GETMRL/GETMWL. Device drivers then use i3c_device_get_info()
       as-is and cannot forget.
    b) In each device driver: the core keeps reporting the raw target
       values, and drivers do the min themselves.
2) Either way, a driver that ignores these limits still gets
    -EOPNOTSUPP from dw_i3c_master_i3c_xfers() when the transfer does
    not fit the data FIFO. Should the driver keep returning that, or
    would you consider splitting an oversized private SDR transfer into
    FIFO-sized chunks in the controller driver? My understanding is
    no - splitting changes what the target sees on the bus - but I
    want to be sure before v2.

Thanks,
Shubham>
>>
>> GETMRL is variable length: the optional third byte is max IBI payload
>> and is only present if the target returned it. Clamp that IBI byte to
>> the IBI queue depth from QUEUE_SIZE_CAPABILITY.IBI_BUF_SIZE (bits 19:16
>> at 0xe8, encoded as 2^(n+1) dwords).
>>
>> Rename the unused EXTENDED_CAPABILITY macro at 0xe8 to the databook
>> name QUEUE_SIZE_CAPABILITY.
>>
>> Signed-off-by: Shubham Patil <shubhamsanjay.patil@amd.com>
>> ---
>>   drivers/i3c/master/dw-i3c-master.c | 149 ++++++++++++++++++++++++++++-
>>   drivers/i3c/master/dw-i3c-master.h |   1 +
>>   2 files changed, 149 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/i3c/master/dw-i3c-master.c b/drivers/i3c/master/dw-i3c-master.c
>> index 4563d8761ba0..51defcb57761 100644
>> --- a/drivers/i3c/master/dw-i3c-master.c
>> +++ b/drivers/i3c/master/dw-i3c-master.c
>> @@ -203,7 +203,13 @@
>>   #define BUS_IDLE_TIMING                      0xd8
>>   #define I3C_VER_ID                   0xe0
>>   #define I3C_VER_TYPE                 0xe4
>> -#define EXTENDED_CAPABILITY          0xe8
>> +#define QUEUE_SIZE_CAPABILITY                0xe8
>> +#define QUEUE_SIZE_CAPABILITY_IBI_BUF(x)     (((x) & GENMASK(19, 16)) >> 16)
>> +/*
>> + * IBI_BUF_SIZE is encoded as 2^(field + 1) dwords: the smallest buffer is
>> + * 2 dwords and each increment of the field doubles the depth.
>> + */
>> +#define QUEUE_SIZE_IBI_BUF_MIN_DWORDS        2
>>   #define SLAVE_CONFIG                 0xec
>>
>>   #define DYN_ADDR_LO_MASK GENMASK(4, 0)
>> @@ -844,6 +850,130 @@ static int dw_i3c_ccc_get(struct dw_i3c_master *master, struct i3c_ccc_cmd *ccc)
>>        return ret;
>>   }
>>
>> +/*
>> + * Cap the limits a target reported through GETMRL to what this controller can
>> + * actually transfer, so the core never asks for a private read the data FIFO
>> + * cannot hold. The optional IBI payload byte is capped to the IBI queue depth
>> + * instead; since that byte is a u8, the IBI cap only ever applies to
>> + * controllers whose IBI queue is smaller than 255 bytes.
>> + *
>> + * Direct SETMRL is optional, so a target may implement GETMRL and NACK the SET.
>> + * Clamp the values handed back to the core either way: a failed SET only means
>> + * the target keeps its own larger limit, which is harmless as long as the core
>> + * stays within ours.
>> + */
>> +static int dw_i3c_master_clamp_mrl(struct dw_i3c_master *master,
>> +                                struct i3c_ccc_cmd *ccc)
>> +{
>> +     u16 max_fifo_bytes = master->caps.datafifodepth * sizeof(u32);
>> +     u32 max_ibi_bytes = master->caps.ibififodepth * sizeof(u32);
>> +     u16 actual_len = ccc->dests[0].payload.actual_len;
>> +     struct i3c_ccc_cmd_dest set_dest = { };
>> +     struct i3c_ccc_cmd set_cmd = { };
>> +     struct i3c_ccc_mrl set_mrl;
>> +     struct i3c_ccc_mrl *mrl;
>> +     bool clamp_ibi = false;
>> +     bool clamp_read;
>> +     u8 ibi_len = 0;
>> +     u16 read_len;
>> +     int ret;
>> +
>> +     /* Need at least the 2-byte max read length field to act on. */
>> +     if (actual_len < 2)
>> +             return 0;
>> +
>> +     mrl = ccc->dests[0].payload.data;
>> +     read_len = be16_to_cpu(mrl->read_len);
>> +     clamp_read = read_len > max_fifo_bytes;
>> +
>> +     /* Optional third byte is valid only if the target returned it. */
>> +     if (actual_len > 2) {
>> +             ibi_len = mrl->ibi_len;
>> +             clamp_ibi = max_ibi_bytes && ibi_len > max_ibi_bytes;
>> +     }
>> +
>> +     if (!clamp_read && !clamp_ibi)
>> +             return 0;
>> +
>> +     set_mrl.read_len = cpu_to_be16(clamp_read ? max_fifo_bytes : read_len);
>> +     if (actual_len > 2)
>> +             set_mrl.ibi_len = clamp_ibi ? max_ibi_bytes : ibi_len;
>> +
>> +     set_dest.addr = ccc->dests[0].addr;
>> +     set_dest.payload.data = &set_mrl;
>> +     set_dest.payload.len = actual_len;
>> +
>> +     set_cmd.rnw = 0;
>> +     set_cmd.id = I3C_CCC_SETMRL(false);
>> +     set_cmd.ndests = 1;
>> +     set_cmd.dests = &set_dest;
>> +
>> +     ret = dw_i3c_ccc_set(master, &set_cmd);
>> +     if (ret)
>> +             dev_dbg(&master->base.dev,
>> +                     "SETMRL not accepted by target: %d\n", ret);
>> +
>> +     if (clamp_read) {
>> +             mrl->read_len = cpu_to_be16(max_fifo_bytes);
>> +             dev_dbg(&master->base.dev,
>> +                     "clamped target MRL from %u to %u bytes (FIFO depth limit)\n",
>> +                     read_len, max_fifo_bytes);
>> +     }
>> +     if (clamp_ibi) {
>> +             mrl->ibi_len = max_ibi_bytes;
>> +             dev_dbg(&master->base.dev,
>> +                     "clamped target IBI len from %u to %u bytes (IBI buffer limit)\n",
>> +                     ibi_len, max_ibi_bytes);
>> +     }
>> +
>> +     return 0;
>> +}
>> +
>> +/* Same contract as dw_i3c_master_clamp_mrl(), for the write direction. */
>> +static int dw_i3c_master_clamp_mwl(struct dw_i3c_master *master,
>> +                                struct i3c_ccc_cmd *ccc)
>> +{
>> +     u16 max_fifo_bytes = master->caps.datafifodepth * sizeof(u32);
>> +     struct i3c_ccc_cmd_dest set_dest = { };
>> +     struct i3c_ccc_cmd set_cmd = { };
>> +     struct i3c_ccc_mwl set_mwl;
>> +     struct i3c_ccc_mwl *mwl;
>> +     u16 write_len;
>> +     int ret;
>> +
>> +     if (ccc->dests[0].payload.actual_len < 2)
>> +             return 0;
>> +
>> +     mwl = ccc->dests[0].payload.data;
>> +     write_len = be16_to_cpu(mwl->len);
>> +
>> +     if (write_len <= max_fifo_bytes)
>> +             return 0;
>> +
>> +     set_mwl.len = cpu_to_be16(max_fifo_bytes);
>> +
>> +     set_dest.addr = ccc->dests[0].addr;
>> +     set_dest.payload.data = &set_mwl;
>> +     set_dest.payload.len = sizeof(set_mwl);
>> +
>> +     set_cmd.rnw = 0;
>> +     set_cmd.id = I3C_CCC_SETMWL(false);
>> +     set_cmd.ndests = 1;
>> +     set_cmd.dests = &set_dest;
>> +
>> +     ret = dw_i3c_ccc_set(master, &set_cmd);
>> +     if (ret)
>> +             dev_dbg(&master->base.dev,
>> +                     "SETMWL not accepted by target: %d\n", ret);
>> +
>> +     mwl->len = cpu_to_be16(max_fifo_bytes);
>> +     dev_dbg(&master->base.dev,
>> +             "clamped target MWL from %u to %u bytes (FIFO depth limit)\n",
>> +             write_len, max_fifo_bytes);
>> +
>> +     return 0;
>> +}
>> +
>>   static int dw_i3c_master_send_ccc_cmd(struct i3c_master_controller *m,
>>                                      struct i3c_ccc_cmd *ccc)
>>   {
>> @@ -866,6 +996,18 @@ static int dw_i3c_master_send_ccc_cmd(struct i3c_master_controller *m,
>>        else
>>                ret = dw_i3c_ccc_set(master, ccc);
>>
>> +     /*
>> +      * Clamp GETMRL/GETMWL responses to the data FIFO depth, and the
>> +      * optional GETMRL IBI byte to the IBI queue depth. The GET itself has
>> +      * already succeeded, so its result is never overridden here.
>> +      */
>> +     if (!ret && ccc->rnw) {
>> +             if (ccc->id == I3C_CCC_GETMRL)
>> +                     dw_i3c_master_clamp_mrl(master, ccc);
>> +             else if (ccc->id == I3C_CCC_GETMWL)
>> +                     dw_i3c_master_clamp_mwl(master, ccc);
>> +     }
>> +
>>        pm_runtime_put_autosuspend(master->dev);
>>        return ret;
>>   }
>> @@ -1728,6 +1870,11 @@ int dw_i3c_common_probe(struct dw_i3c_master *master,
>>        ret = readl(master->regs + DATA_BUFFER_STATUS_LEVEL);
>>        master->caps.datafifodepth = DATA_BUFFER_STATUS_LEVEL_TX(ret);
>>
>> +     /* Read the IBI data buffer size advertised by the controller. */
>> +     ret = readl(master->regs + QUEUE_SIZE_CAPABILITY);
>> +     master->caps.ibififodepth = QUEUE_SIZE_IBI_BUF_MIN_DWORDS <<
>> +                                 QUEUE_SIZE_CAPABILITY_IBI_BUF(ret);
>> +
>>        ret = readl(master->regs + DEVICE_ADDR_TABLE_POINTER);
>>        master->datstartaddr = ret;
>>        master->maxdevs = ret >> 16;
>> diff --git a/drivers/i3c/master/dw-i3c-master.h b/drivers/i3c/master/dw-i3c-master.h
>> index 17ad817d1f8e..54c3912374c8 100644
>> --- a/drivers/i3c/master/dw-i3c-master.h
>> +++ b/drivers/i3c/master/dw-i3c-master.h
>> @@ -15,6 +15,7 @@
>>   struct dw_i3c_master_caps {
>>        u8 cmdfifodepth;
>>        u8 datafifodepth;
>> +     u32 ibififodepth;
>>   };
>>
>>   struct dw_i3c_dat_entry {
>> --
>> 2.34.1
>>