Re: [PATCH] ata: Add sdev attribute to lower link speed in runtime

[Gustav Ekelund]

On 4/17/24 12:14, Niklas Cassel wrote:
> On Mon, Apr 15, 2024 at 04:49:46PM +0200, Gustav Ekelund wrote:
>> On 4/13/24 02:29, Damien Le Moal wrote:
>>> On 4/12/24 22:48, Gustav Ekelund wrote:
>>>> Expose a new sysfs attribute to userspace that gives root the ability to
>>>> lower the link speed in a scsi_device at runtime. The handle enables
>>>> programs to, based on external circumstances that may be unbeknownst to
>>>> the kernel, determine if a link should slow down to perhaps achieve a
>>>> stabler signal. External circumstances could include the mission time
>>>> of the connected hardware or observations to temperature trends.
>>>
>>> may, perhaps, could... This does not sound very deterministic. Do you have an
>>> actual practical use case where this patch is useful and solve a real problem ?
>>>
>>> Strictly speaking, if you are seeing link stability issues due to temperature or
>>> other environmental factors (humidity, altitude), then either you are operating
>>> your hardware (board and/or HDD) outside of their environmental specifications,
>>> or you have some serious hardware issues (which can be a simple as a bad SATA
>>> cable or an inappropriate power supply). In both cases, I do not think that this
>>> patch will be of any help.
>>>
>>> Furthermore, libata already lowers a link speed automatically at runtime if it
>>> sees too many NCQ errors. Isn't that enough ? And we also have the horkage flags
>>> to force a maximum link speed for a device/adapter, which can also be specified
>>> as a libata module argument (libata.force).
>>>
>>>> Writing 1 to /sys/block/*/device/down_link_spd signals the kernel to
>>>> first lower the link speed one step with sata_down_spd_limit and then
>>>> finish off with sata_link_hardreset.
>>>
>>> We already have "/sys/class/ata_link/*/hw_sata_spd_limit", which is read-only
>>> for now. So if you can really justify this manual link speed tuning for an
>>> actual use case (not a hypothetical one), then the way to go would be to make
>>> that attribute RW and implement its store() method to lower the link speed at
>>> runtime.
>>>
>>> And by the way, looking at what that attribute says, I always get:
>>> <unknown>
>>>
>>> So it looks like there is an issue with it that went unnoticed (because no one
>>> is using it...). This needs some fixing.
>>>
>> Hello Damien and Niklas,
>>
>> Thank you for the feedback.
>>
>> I have a hotplug system, where the links behave differently depending
>> on the disk model connected. For some models the kernel emits a lot of
>> bus errors, but mostly not enough errors for it to automatically lower
>> the link speed, except during high workloads. I have not observed any
>> data-loss regarding the errors, but the excessive logging becomes a problem.
> 
> It might be interesting to compare the output of:
> $ hdparm -I
> 
> for a drive that you can hot plug insert without errors, against a drive
> that gives you errors on hot plug insertion, to see if this can give you
> a hint of why they behave differently.
> 
> (e.g. certain features, e.g. DevSleep, is only enabled if there is support
> in the HBA, the port, and the drive.)
> 
> 
> Kind regards,
> Niklas
Hi Niklas,

I mostly tested on the 6.1 kernel, and it is a quite peculiar hardware
problem, so it isn't caused by anythin in the latest 6.9 rc.

Thank you for the advice of using hdparm, maybe I can diff preset of
features between the models. I will share any interesting findings I
come across.

Best regards
Gustav