Re: [PATCH v6 1/3] scsi: ufs: Allow RTT negotiation

[Bart Van Assche <bvanassche@acm.org>]

On 5/26/24 01:16, Avri Altman wrote:
> The rtt-upiu packets precede any data-out upiu packets, thus
> synchronizing the data input to the device: this mostly applies to write
> operations, but there are other operations that requires rtt as well.
> 
> There are several rules binding this rtt - data-out dialog, specifically
> There can be at most outstanding bMaxNumOfRTT such packets.  This might
> have an effect on write performance (sequential write in particular), as
> each data-out upiu must wait for its rtt sibling.
> 
> UFSHCI expects bMaxNumOfRTT to be min(bDeviceRTTCap, NORTT). However,
> as of today, there does not appears to be no-one who sets it: not the
> host controller nor the driver.  It wasn't an issue up to now:
> bMaxNumOfRTT is set to 2 after manufacturing, and wasn't limiting the
> write performance.
> 
> UFS4.0, and specifically gear 5 changes this, and requires the device to
> be more attentive.  This doesn't come free - the device has to allocate
> more resources to that end, but the sequential write performance
> improvement is significant. Early measurements shows 25% gain when
> moving from rtt 2 to 9. Therefore, set bMaxNumOfRTT to be
> min(bDeviceRTTCap, NORTT) as UFSHCI expects.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>