Re: [PATCH] coresight: Defer probe when the child dev is not probed

[Mike Leach]

Hi,

On Tue, 1 Mar 2022 at 11:42, Jinlong Mao <quic_jinlmao@xxxxxxxxxxx> wrote:
>
> On 2/28/2022 10:51 PM, Suzuki K Poulose wrote:
>
> Hi Jinlong
>
> On 28/02/2022 13:31, Mao Jinlong wrote:
>
> From: Mao Jinlong <jinlmao@xxxxxxxxxxxxxxxx>
>
> It is possible that when device probe, its child device is not
> probed. Then it will fail when add sysfs connection for the device.
> Make device defer probe when the child device is not probed.
>
>
> Please could you a bit a more specific on the exact issue ?
> I don't see a problem with probe deferral right now, with
> coresight/next.
>
> For e.g.,
>
> root@juno-server:~# lsmod
> Module                  Size  Used by
> coresight              73728  0
> root@juno-server:~# ls /sys/bus/coresight/devices/
> root@juno-server:~# modprobe coresight-etm4x
> root@juno-server:~# lsmod
> Module                  Size  Used by
> coresight_etm4x        81920  0
> coresight              73728  1 coresight_etm4x
> root@juno-server:~# ls /sys/bus/coresight/devices/
> etm0  etm1
>
> -- Note etm2-etm5 doesn't appear --
>
> root@juno-server:~# modprobe coresight-funnel
> root@juno-server:~# lsmod
> Module                  Size  Used by
> coresight_funnel       20480  0
> coresight_etm4x        81920  0
> coresight              73728  2 coresight_etm4x,coresight_funnel
> root@juno-server:~# ls /sys/bus/coresight/devices/
> etm0  etm1
>
> -- Still don't appear ---
>
> root@juno-server:~# modprobe coresight-replicator
> root@juno-server:~# ls /sys/bus/coresight/devices/
> etm0  etm1
> root@juno-server:~# modprobe coresight-tmc
>
> -- At this stage, the devices automatically get probed and appear --
> root@juno-server:~# ls /sys/bus/coresight/devices/
> etm0  etm1  etm2  etm3  etm4  etm5  funnel0  funnel1  funnel2  tmc_etf0  tmc_etr0
>
>
> root@juno-server:~# lsmod
> Module                  Size  Used by
> coresight_tmc          40960  0
> coresight_replicator    20480  0
> coresight_funnel       20480  0
> coresight_etm4x        81920  0
> coresight              73728  4 coresight_tmc,coresight_etm4x,coresight_replicator,coresight_funnel
>
> So, my question is, what is this patch trying to solve ?
>
>
> Cheers
> Suzuki
>
> Hi Suzuki,
>
> This issue happens when race condition happens.
> The condition is that the device and its child_device's probe happens at the same time.
>
> For example: device0 and its child device device1.
> Both of them are calling coresight_register function. device0 is calling coresight_fixup_device_conns.
> device1 is waiting for device0 to release the coresight_mutex. Because device1's csdev node is allocated,
> coresight_make_links will be called for device0. Then in coresight_add_sysfs_link, has_conns_grp is true
> for device0, but has_conns_grp is false for device1 as has_conns_grp is set to true in coresight_create_conns_sysfs_group .
> The probe of device0 will fail for at this condition.
>
>
> struct coresight_device *coresight_register(struct coresight_desc *desc)
> {
>    .........
>     mutex_lock(&coresight_mutex);
>
>     ret = coresight_create_conns_sysfs_group(csdev);
>     if (!ret)
>         ret = coresight_fixup_device_conns(csdev);
>     if (!ret)
>         ret = coresight_fixup_orphan_conns(csdev);
>     if (!ret && cti_assoc_ops && cti_assoc_ops->add)
>         cti_assoc_ops->add(csdev);
>
>     mutex_unlock(&coresight_mutex);
>
> .........
>
> }
>
> static int coresight_fixup_device_conns(struct coresight_device *csdev)
> {
>    ..........
>         conn->child_dev =
>             coresight_find_csdev_by_fwnode(conn->child_fwnode);

The issue appears to be a constraint hidden in the lower layers of the code.
Would a better solution not be to alter the code here:

if (conn->child_dev && conn->child_dev->has_conns_grp) {
   ...
} else {
      csdev->orphan = true;
}

which would mean that the connection attempt would drop through to
label the connection as an orphan, to be cleaned up by the child
itself when it runs coresight_fixup_orphan_conns()

Regards

Mike

>         if (conn->child_dev) {
>             ret = coresight_make_links(csdev, conn,
>
>                            conn->child_dev);
>
> ..........
>
> }
>
>
> int coresight_add_sysfs_link(struct coresight_sysfs_link *info)
> {
>    ................
>     if (!info->orig->has_conns_grp || !info->target->has_conns_grp)
>         return -EINVAL;
>
>
>
> The probe fail issue is reproduced with reboot stress test on our internal device.
>
> With the patch, the probe fail issue is not reproduced.
>
> Thanks
>
> Jinlong Mao
>
>
>
> Signed-off-by: Mao Jinlong <jinlmao@xxxxxxxxxxxxxxxx>
> ---
>   drivers/hwtracing/coresight/coresight-sysfs.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-sysfs.c b/drivers/hwtracing/coresight/coresight-sysfs.c
> index 34d2a2d31d00..7df9eb59bf2c 100644
> --- a/drivers/hwtracing/coresight/coresight-sysfs.c
> +++ b/drivers/hwtracing/coresight/coresight-sysfs.c
> @@ -73,8 +73,10 @@ int coresight_add_sysfs_link(struct coresight_sysfs_link *info)
>       if (!info->orig || !info->target ||
>           !info->orig_name || !info->target_name)
>           return -EINVAL;
> -    if (!info->orig->has_conns_grp || !info->target->has_conns_grp)
> +    if (!info->orig->has_conns_grp)
>           return -EINVAL;
> +    if (!info->target->has_conns_grp)
> +        return -EPROBE_DEFER;
>         /* first link orig->target */
>       ret = sysfs_add_link_to_group(&info->orig->dev.kobj,
>
>


-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK