[v3,2/9] ide: Do not set the RQF_PREEMPT flag for sense requests

RQF_PREEMPT is used for two different purposes in the legacy IDE code:
1. To mark power management requests.
2. To mark requests that should preempt another request. An (old)
   explanation of that feature is as follows:
   "The IDE driver in the Linux kernel normally uses a series of busywait
   delays during its initialization. When the driver executes these
   busywaits, the kernel does nothing for the duration of the wait. The
   time spent in these waits could be used for other initialization
   activities, if they could be run concurrently with these waits.

   More specifically, busywait-style delays such as udelay() in module
   init functions inhibit kernel preemption because the Big Kernel Lock
   is held, while yielding APIs such as schedule_timeout() allow preemption.
   This is true because the kernel handles the BKL specially and releases
   and reacquires it across reschedules allowed by the current thread.

   This IDE-preempt specification requires that the driver eliminate these
   busywaits and replace them with a mechanism that allows other work to
   proceed while the IDE driver is initializing."

Since I haven't found an implementation of (2), do not set the PREEMPT
flag for sense requests. This patch causes sense requests to be
postponed while a drive is suspended instead of being submitted to
ide_queue_rq().

If it would ever be necessary to restore the IDE PREEMPT functionality,
that can be done by introducing a new flag in struct ide_request.

This patch is a first step towards removing the PREEMPT flag from the
block layer.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Can Guo <cang@codeaurora.org>
Cc: Stanley Chu <stanley.chu@mediatek.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/ide/ide-atapi.c | 1 -
 1 file changed, 1 deletion(-)

On 11/23/20 4:17 AM, Bart Van Assche wrote:
> RQF_PREEMPT is used for two different purposes in the legacy IDE code:
> 1. To mark power management requests.
> 2. To mark requests that should preempt another request. An (old)
>     explanation of that feature is as follows:
>     "The IDE driver in the Linux kernel normally uses a series of busywait
>     delays during its initialization. When the driver executes these
>     busywaits, the kernel does nothing for the duration of the wait. The
>     time spent in these waits could be used for other initialization
>     activities, if they could be run concurrently with these waits.
> 
>     More specifically, busywait-style delays such as udelay() in module
>     init functions inhibit kernel preemption because the Big Kernel Lock
>     is held, while yielding APIs such as schedule_timeout() allow preemption.
>     This is true because the kernel handles the BKL specially and releases
>     and reacquires it across reschedules allowed by the current thread.
> 
>     This IDE-preempt specification requires that the driver eliminate these
>     busywaits and replace them with a mechanism that allows other work to
>     proceed while the IDE driver is initializing."
> 
> Since I haven't found an implementation of (2), do not set the PREEMPT
> flag for sense requests. This patch causes sense requests to be
> postponed while a drive is suspended instead of being submitted to
> ide_queue_rq().
> 
> If it would ever be necessary to restore the IDE PREEMPT functionality,
> that can be done by introducing a new flag in struct ide_request.
> 
> This patch is a first step towards removing the PREEMPT flag from the
> block layer.
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: Can Guo <cang@codeaurora.org>
> Cc: Stanley Chu <stanley.chu@mediatek.com>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>   drivers/ide/ide-atapi.c | 1 -
>   1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/ide/ide-atapi.c b/drivers/ide/ide-atapi.c
> index 2162bc80f09e..013ad33fbbc8 100644
> --- a/drivers/ide/ide-atapi.c
> +++ b/drivers/ide/ide-atapi.c
> @@ -223,7 +223,6 @@ void ide_prep_sense(ide_drive_t *drive, struct request *rq)
>   	sense_rq->rq_disk = rq->rq_disk;
>   	sense_rq->cmd_flags = REQ_OP_DRV_IN;
>   	ide_req(sense_rq)->type = ATA_PRIV_SENSE;
> -	sense_rq->rq_flags |= RQF_PREEMPT;
>   
>   	req->cmd[0] = GPCMD_REQUEST_SENSE;
>   	req->cmd[4] = cmd_len;
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes