linux-kernel - Re: [PATCH v3 25/77] ncr5380: Rework disconnect versus poll logic

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Date:	Tue, 22 Dec 2015 08:21:22 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Finn Thain <fthain@...egraphics.com.au>,
	"James E.J. Bottomley" <JBottomley@...n.com>,
	Michael Schmitz <schmitzmic@...il.com>,
	linux-m68k@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH v3 25/77] ncr5380: Rework disconnect versus poll logic

On 12/22/2015 02:18 AM, Finn Thain wrote:
> The atari_NCR5380.c and NCR5380.c core drivers differ in their handling of
> target disconnection. This is partly because atari_NCR5380.c had all of
> the polling and sleeping removed to become entirely interrupt-driven, and
> it is partly because of damage done to NCR5380.c after atari_NCR5380.c was
> forked. See commit 37cd23b44929 ("Linux 2.1.105") in history/history.git.
>
> The polling changes that were made in v2.1.105 are questionable at best:
> if REQ is not already asserted when NCR5380_transfer_pio() is invoked, and
> if the expected phase is DATA IN or DATA OUT, the function will schedule
> main() to execute after USLEEP_SLEEP jiffies and then return. The problems
> here are the expected REQ timing and the sleep interval*. Avoid this issue
> by using NCR5380_poll_politely() instead of scheduling main().
>
> The atari_NCR5380.c core driver requires the use of the chip interrupt and
> always permits target disconnection. It sets the cmd->device->disconnect
> flag when a device disconnects, but never tests this flag.
>
> The NCR5380.c core driver permits disconnection only when
> instance->irq != NO_IRQ. It sets the cmd->device->disconnect flag when
> a device disconnects and it tests this flag in a couple of places:
>
> 1. During NCR5380_information_transfer(), following COMMAND OUT phase,
>     if !cmd->device->disconnect, the initiator will take a guess as to
>     whether or not the target will then choose to go to MESSAGE IN phase
>     and disconnect. If the driver guesses "yes", it will schedule main()
>     to execute after USLEEP_SLEEP jiffies and then return there.
>
>     Unfortunately the driver may guess "yes" even after it has denied
>     the target the disconnection privilege. When the target does not
>     disconnect, the sleep can be beneficial, assuming the sleep interval
>     is appropriate (mostly it is not*).
>
>     And even if the driver guesses "yes" correctly, and the target would
>     then disconnect, the driver still has to go through the MESSAGE IN
>     phase in order to get to BUS FREE phase. The main loop can do nothing
>     useful until BUS FREE, and sleeping just delays the phase transition.
>
> 2. If !cmd->device->disconnect and REQ is not already asserted when
>     NCR5380_information_transfer() is invoked, the function polls for REQ
>     for USLEEP_POLL jiffies. If REQ is not asserted, it then schedules
>     main() to execute after USLEEP_SLEEP jiffies and returns.
>
>     The idea is apparently to yeild the CPU while waiting for REQ.
>     This is conditional upon !cmd->device->disconnect, but there seems
>     to be no rhyme or reason for that. For example, the flag may be
>     unset because disconnection privilege was denied because the driver
>     has no IRQ. Or the flag may be unset because the device has never
>     needed to disconnect before. Or if the flag is set, disconnection
>     may have no relevance to the present bus phase.
>
> Another deficiency of the existing algorithm is as follows. When the
> driver has no IRQ, it prevents disconnection, and generally polls and
> sleeps more than it would normally. Now, if the driver is going to poll
> anyway, why not allow the target to disconnect? That way the driver can do
> something useful with the bus instead of polling unproductively!
>
> Avoid this pointless latency, complexity and guesswork by using
> NCR5380_poll_politely() instead of scheduling main().
>
> * For g_NCR5380, the time intervals for USLEEP_SLEEP and USLEEP_POLL are
>    200 ms and 10 ms, respectively. They are 20 ms and 200 ms respectively
>    for the other NCR5380 drivers. There doesn't seem to be any reason for
>    this discrepancy. The timing seems to have no relation to the type of
>    adapter. Bizarrely, the timing in g_NCR5380 seems to relate only to one
>    particular type of target device. This patch attempts to solve the
>    problem for all NCR5380 drivers and all target devices.
>
> Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
>
> ---
>   drivers/scsi/NCR5380.c       |  137 ++-----------------------------------------
>   drivers/scsi/NCR5380.h       |   11 ---
>   drivers/scsi/atari_NCR5380.c |   24 ++-----
>   drivers/scsi/g_NCR5380.c     |    4 -
>   4 files changed, 15 insertions(+), 161 deletions(-)
>
Reviewed-by: Hannes Reinecke <hare@...e.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@...e.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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