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

[Gustav Ekelund <gustaek@axis.com>]

On 4/17/24 00:59, Damien Le Moal wrote:
> On 2024/04/16 0:49, 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.
> 
> Hot-plugging should not be an issue in itself. When hot-plugged, the port
> scanning process should detect the maximum link speed supported by your device
> and use that speed for probing the device itself (IDENTIFY etc). If you see bus
> errors, then you are either having hardware issues (e.g. a bad cable or power
> supply) or some issues with your AHCI controller that may need patching.
> 
> Can you send examples of the errors you are seeing ? That needs to be
> investigated first before going the (drastic) route of allowing to manually
> lower link speed at run-time.
> 
>>
>> So I want to adapt the link, depending on the connected model, in a
>> running system because I know that some particular models in this case
>> will operate better in SATA2 in this system.
>>
>> Can I use the libata.force module to make changes to a particular link
>> in runtime?
> 
> Nope, libata.force is a module parameter so you can specify it as a kernel boot
> parameter, or if you compile libata as a module when loading (modprobe) libata.
> At run time, you need to rmmod+modprobe again libata, and so the ahci driver as
> well (because of dependencies).
> 
> As I mentioned, if a run-time knob really is necessary (it should not be), using
> the ata_link hw_sata_spd_limit would be a better approach. But again, that
> really should not be necessary at all.
> 
>>
>> Best regards
>> Gustav
>>
> 
Hi Damien,

Unfortunately doing this selectively per link from user space seems to
be my only way forward for this particular hardware issue. I understand
if you deem this to be too specific to fit into the upstream kernel.

I will investigate if running libata as a module might be a way around
this peculiar problem. If I need to refine my first approach, I will
attempt to modify the hw_sata_spd_limit to be rw.

Thank you Damien and Niklas.

Best regards
Gustav