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
