[PATCH 04/18] mmc: meson-gx: improve meson_mmc_start_cmd

[hkallweit1@gmail.com (Heiner Kallweit)]

Am 15.02.2017 um 20:43 schrieb Heiner Kallweit:
> Am 15.02.2017 um 18:04 schrieb Kevin Hilman:
>> Heiner Kallweit <hkallweit1@gmail.com> writes:
>>
>>> Remove use of unneeded members cmd_arg and cmd_resp.
>>> Setting SD_EMMC_CMD_RSP is only needed if CMD_CFG_RESP_NUM is set,
>>> so don't write this register in all other cases.
>>>
>>> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
>>
>> I'm not sure I like this change.  This works now because there is only
>> one descriptor used, but one of the next things to work on in this
>> driver is taking advantage of the internal DMA capabilities, which means
>> having a chain of descriptorsall filled out in memory.
>>
> For testing purposes I temporarily changed the driver from passing the
> descriptor in registers to passing the descriptor via DMA and it worked.
> 
> I'm not very familiar (yet) with descriptor chains and have to check
> the MMC core code a little bit more ..
> If we can actually benefit from it then I'd agree with you.
> 
After having had a little bit closer look at the MMC core and descriptor
chains in general:
I'm not really sure where descriptor chains would help us. SG lists in
read / write commands are linearized by the driver via
sg_copy_[to,from]_buffer already.
Beyond that I don't see any command chaining support in the core
(prerequisite for command chaining most likely would be that a
subsequent command must not depend on the response of a previous one).

Maybe somebody with more knowledge about the MMC core and MMC in general
can shed some light on this ..

Heiner
> 
>> Kevin
>>
>>> ---
>>>  drivers/mmc/host/meson-gx-mmc.c | 6 ++----
>>>  1 file changed, 2 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/mmc/host/meson-gx-mmc.c b/drivers/mmc/host/meson-gx-mmc.c
>>> index ece38b44..630e0590 100644
>>> --- a/drivers/mmc/host/meson-gx-mmc.c
>>> +++ b/drivers/mmc/host/meson-gx-mmc.c
>>> @@ -456,7 +456,6 @@ static void meson_mmc_start_cmd(struct mmc_host *mmc, struct mmc_command *cmd)
>>>  	desc->cmd_cfg |= (cmd->opcode & CMD_CFG_CMD_INDEX_MASK)	<<
>>>  		CMD_CFG_CMD_INDEX_SHIFT;
>>>  	desc->cmd_cfg |= CMD_CFG_OWNER;  /* owned by CPU */
>>> -	desc->cmd_arg = cmd->arg;
>>>  
>>>  	/* Response */
>>>  	if (cmd->flags & MMC_RSP_PRESENT) {
>>> @@ -464,7 +463,7 @@ static void meson_mmc_start_cmd(struct mmc_host *mmc, struct mmc_command *cmd)
>>>  		if (cmd->flags & MMC_RSP_136)
>>>  			desc->cmd_cfg |= CMD_CFG_RESP_128;
>>>  		desc->cmd_cfg |= CMD_CFG_RESP_NUM;
>>> -		desc->cmd_resp = 0;
>>> +		writel(0, host->regs + SD_EMMC_CMD_RSP);
>>>  
>>>  		if (!(cmd->flags & MMC_RSP_CRC))
>>>  			desc->cmd_cfg |= CMD_CFG_RESP_NOCRC;
>>> @@ -540,9 +539,8 @@ static void meson_mmc_start_cmd(struct mmc_host *mmc, struct mmc_command *cmd)
>>>  	desc->cmd_cfg |= CMD_CFG_END_OF_CHAIN;
>>>  	writel(desc->cmd_cfg, host->regs + SD_EMMC_CMD_CFG);
>>>  	writel(desc->cmd_data, host->regs + SD_EMMC_CMD_DAT);
>>> -	writel(desc->cmd_resp, host->regs + SD_EMMC_CMD_RSP);
>>>  	wmb(); /* ensure descriptor is written before kicked */
>>> -	writel(desc->cmd_arg, host->regs + SD_EMMC_CMD_ARG);
>>> +	writel(cmd->arg, host->regs + SD_EMMC_CMD_ARG);
>>>  }
>>>  
>>>  static void meson_mmc_request(struct mmc_host *mmc, struct mmc_request *mrq)
>>
>