Re: [PATCH] libata: sata_down_spd_limit should return if driver has not recorded sstatus speed

David Milburn
2017-11-27T20:54:03Z

Hi Tejun,

On 11/27/2017 01:19 PM, Tejun Heo wrote:
> Hello, David.
>
> On Tue, Nov 14, 2017 at 04:17:25PM -0600, David Milburn wrote:
>> During hotplug, it is possible for 6Gbps link speed to
>> be limited all the way down to 1.5 Gbps which may lead
>> to a slower link speed when drive is re-connected.
>
> Can you please explain the scenario with more details?

Yes, in this particular case (I had a second reporter, but,
they didn't respond to my questions), when disconnecting
the drive, the error handler limits the speed to 1.5 Gpps
by setting bit 4 of the control register. And when the drive
is reconnected the status register show a link speed of 3Gbps,
even though before hot unplug it was running a 6Gbps.

>
>> /* Mask off all speeds higher than or equal to the current
>> - * one. Force 1.5Gbps if current SPD is not available.
>> + * one. At this point if current SPD is not available and we
>> + * previously recorded the link speed from the Status register,
>> + * the driver has already masked off the highest bit so mask
>> + * should already be set to 1 or 0. Otherwise, we should
>> + * not force 1.5Gbps on a link where we have not previously
>> + * recorded speed from Status register, just return in this case.
>> */
>> if (spd > 1)
>> mask &= (1 << (spd - 1)) - 1;
>> else
>> - mask &= 1;
>> + return -EINVAL;
>
> I get that the current behavior might not be ideal in certain
> scenarios but the above seems weird, especially given the we have
> link->sata_spd for cases where we can't access the spd value.
>

Yes, but link->sata_spd is initialized to 0, and the only time the
driver records "link->sata_spd" from the status register is in
ata_eh_reset(). And on hot unplug, the driver sets the mask to 1
dropping the link from 6 to 1.5Gbps. And, if the driver doesn't
downgrade the link speed by writing to the control register, the link
comes back at 6Gbps on hot plug. It maybe particular controller/configs
behave differently since it is not always reproducible.

Thanks,
David