RE: RE: [RESEND PATCH] scsi: ufs: sysfs: support writing boot_lun attr

[Avri Altman <Avri.Altman@wdc.com>]

Hi Caleb,

> On 27/05/2022 07:17, Avri Altman wrote:
> >>
> >> Hi,
> >>
> >> My usecase is enabling boot slot switching on Android A/B devices,
> >> where the active LUN has to be changed in order to facilitate e.g.
> >> dual-booting Android and mainline Linux. A similar interface is
> >> exposed by Android, albeit via ioctl. I've tested this patch and confirmed it
> enabled the necessary functionality.
> >>
> >> On 25/05/2022 21:34, Avri Altman wrote:
> >>> Hi,
> >>>> Expands sysfs boot_lun attribute to be writable. Necessary to
> >>>> enable proper support for LUN switching on some UFS devices.
> >>> Can you please elaborate why is it necessary?
> >>> What use case are you running?
> > NAK with prejudice.
> Hi Avri,
> 
> Could you explain why the NAK here? Boot LUN switching is used on a lot of
> embedded devices to implement A/B updates, Android devices are just one such
> example.
Sorry for not giving a proper explanation.
As a design rule, sysfs attribute files should not be used to make persistent modifications
to a device configuration. This rule applies to all subsystems and ufs is no different.

> 
> Distributions like postmarketOS [1] aim to support upstream Linux on mobile
> devices, particularly those that are no longer supported by the vendor. Being
> able to make use of features like A/B updates is something that I expect more
> distributions to be considering in the future, as we start to see more Linux
> devices with support for features like this.
Changing a UFS device configuration can be done using the ufs-bsg framework.
This framework provides a command passthrough interface with the /dev/ufs-bsgX
bsg node. For examples on how to use this, you can see the ufs-utils project:
https://github.com/westerndigitalcorporation/ufs-utils

Thanks,
Avri

> 
> If safety is a concern, or if the values are device specific, we can look at
> protecting the write functionality and configuration behind DT properties, or
> coming up with another alternative.
> 
> [1]: https://postmarketos.org
> 
> Kind regards,
> Caleb (they/he)
> 
> >
> > Thanks,
> > Avri
> >
> >>>
> >>>> Signed-off-by: Nia Espera <a5b6@riseup.net>
> >>>> ---
> >>>>    drivers/scsi/ufs/ufs-sysfs.c | 67
> +++++++++++++++++++++++++++++++++++-
> >>>>    1 file changed, 66 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/drivers/scsi/ufs/ufs-sysfs.c
> >>>> b/drivers/scsi/ufs/ufs-sysfs.c index 5c405ff7b6ea..7bf5d6c3d0ec
> >>>> 100644
> >>>> --- a/drivers/scsi/ufs/ufs-sysfs.c
> >>>> +++ b/drivers/scsi/ufs/ufs-sysfs.c
> >>>> @@ -1047,6 +1047,71 @@ static inline bool ufshcd_is_wb_attrs(enum
> >>>> attr_idn
> >>>> idn)
> >>>>                   idn <= QUERY_ATTR_IDN_CURR_WB_BUFF_SIZE;
> >>>>    }
> >>>>
> >>>> +static ssize_t boot_lun_enabled_show(struct device *dev,
> >>>> +                                    struct device_attribute *attr,
> >>>> +char *buf) {
> >>>> +       struct ufs_hba *hba = dev_get_drvdata(dev);
> >>>> +       u32 slot;
> >>>> +       int ret;
> >>>> +       u8 index = 0;
> >>>> +
> >>>> +       down(&hba->host_sem);
> >>>> +       if (!ufshcd_is_user_access_allowed(hba)) {
> >>>> +               up(&hba->host_sem);
> >>>> +               return -EBUSY;
> >>>> +       }
> >>>> +       if (ufshcd_is_wb_attrs(QUERY_ATTR_IDN_BOOT_LU_EN))
> >>> Clearly bBootLunEn is not a WB attribute.
> >>>
> >>>> +               index = ufshcd_wb_get_query_index(hba);
> >>>> +       ufshcd_rpm_get_sync(hba);
> >>>> +
> >>>> +       ret = ufshcd_query_attr(hba, UPIU_QUERY_OPCODE_READ_ATTR,
> >>>> +               QUERY_ATTR_IDN_BOOT_LU_EN, index, 0, &slot);
> >>>> +
> >>>> +       ufshcd_rpm_put_sync(hba);
> >>>> +       if (ret) {
> >>>> +               ret = -EINVAL;
> >>>> +               goto out;
> >>>> +       }
> >>>> +
> >>>> +       ret = sysfs_emit(buf, "0x%08X\n", slot);
> >>>> +out:
> >>>> +       up(&hba->host_sem);
> >>>> +       return ret;
> >>>> +}
> >>>> +
> >>>> +static ssize_t boot_lun_enabled_store(struct device *dev,
> >>>> +                                     struct device_attribute *attr,
> >>>> +                                     const char *buf, size_t
> >>>> +count) {
> >>>> +       struct ufs_hba *hba = dev_get_drvdata(dev);
> >>>> +       u32 slot;
> >>>> +       int ret;
> >>>> +       u8 index = 0;
> >>>> +
> >>>> +       if (kstrtouint(buf, 0, &slot) < 0)
> >>>> +               return -EINVAL;
> >>> You need to verify that no one set bBootLunEn = 0x0 because the
> >>> device won't
> >> boot.
> >>> Better check explicitly that slot != bBootLunEn and its either 1 or 2.
> >>>
> >>> Thanks,
> >>> Avri
> >>>
> >>>> +
> >>>> +       down(&hba->host_sem);
> >>>> +       if (!ufshcd_is_user_access_allowed(hba)) {
> >>>> +               up(&hba->host_sem);
> >>>> +               return -EBUSY;
> >>>> +       }
> >>>> +       if (ufshcd_is_wb_attrs(QUERY_ATTR_IDN_BOOT_LU_EN))
> >>>> +               index = ufshcd_wb_get_query_index(hba);
> >>>> +       ufshcd_rpm_get_sync(hba);
> >>>> +
> >>>> +       ret = ufshcd_query_attr_retry(hba,
> >> UPIU_QUERY_OPCODE_WRITE_ATTR,
> >>>> +                                     QUERY_ATTR_IDN_BOOT_LU_EN, index, 0, &slot);
> >>>> +       ufshcd_rpm_put_sync(hba);
> >>>> +       if (ret) {
> >>>> +               ret = -EINVAL;
> >>>> +               goto out;
> >>>> +       }
> >>>> +out:
> >>>> +       up(&hba->host_sem);
> >>>> +       return ret ? ret : count;
> >>>> +}
> >>>> +
> >>>>    #define UFS_ATTRIBUTE(_name, _uname)                                   \
> >>>>    static ssize_t _name##_show(struct device *dev,                                \
> >>>>           struct device_attribute *attr, char *buf)                       \
> >>>> @@ -1077,8 +1142,8 @@ out:                                                                      \
> >>>>           return ret;                                                     \
> >>>>    }                                                                      \
> >>>>    static DEVICE_ATTR_RO(_name)
> >>>> +static DEVICE_ATTR_RW(boot_lun_enabled);
> >>>>
> >>>> -UFS_ATTRIBUTE(boot_lun_enabled, _BOOT_LU_EN);
> >>>>    UFS_ATTRIBUTE(max_data_size_hpb_single_cmd,
> >> _MAX_HPB_SINGLE_CMD);
> >>>>    UFS_ATTRIBUTE(current_power_mode, _POWER_MODE);
> >>>>    UFS_ATTRIBUTE(active_icc_level, _ACTIVE_ICC_LVL);
> >>>> --
> >>>> 2.36.1