linux-kernel - Re: [PATCH] esp_scsi: Clear Transfer Count registers before PIO transfers

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Message-ID: <CACz-3rjHAyi6kMQ6j9YALLm1ApYrsqKiTnGNPUhxqqEuRJ9TjQ@mail.gmail.com>
Date:   Sun, 17 Nov 2019 07:26:16 +0100
From:   Kars de Jong <jongk@...ux-m68k.org>
To:     Finn Thain <fthain@...egraphics.com.au>
Cc:     "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Michael Schmitz <schmitzmic@...il.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] esp_scsi: Clear Transfer Count registers before PIO transfers

Hi Finn,

Op za 16 nov. 2019 om 04:36 schreef Finn Thain <fthain@...egraphics.com.au>:
>
> The zorro_esp driver uses both PIO and DMA transfers. If a failed DMA
> transfer happened to be followed by a PIO transfer, the TCLOW and TCMED
> registers would not get cleared. It is theoretically possible that the
> stale value from the transfer counter or the TCLOW/TCMED registers
> could then be used by the controller and the driver. Avoid that by
> clearing these registers before each PIO transfer.

Are you sure this is really needed?

The only place where the driver reads these registers is after a data
transfer. These are done using DMA on all Zorro boards, so I don’t
think there’s a risk of stale values from a PIO transfer there.

The only place the controller reads these registers is when a DMA
command is issued. The only place where that is done is in the
zorro_esp send_dma_command() functions. These all set both registers
explicitly before issuing the DMA command to the controller, so I
don’t think there’s a risk there either.

Kind regards,

Kars.