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

[Caleb Connolly]

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.

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.

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@xxxxxxxxxx>
> > > > ---
> > > >    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