linux-kernel - Re: [PATCH 0/1] scsi: qla1280: Make 64-bit DMA addressing a Kconfig option

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Message-ID: <CA+=Fv5TJRpFJ=p1MLcORDaqnSG-0AUEtSUw3Kek0vPGKbQZT9g@mail.gmail.com>
Date: Tue, 5 Aug 2025 10:52:15 +0200
From: Magnus Lindholm <linmag7@...il.com>
To: "Maciej W. Rozycki" <macro@...am.me.uk>
Cc: linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org, 
	linux-alpha@...r.kernel.org, martin.petersen@...cle.com, 
	James.Bottomley@...senpartnership.com, hch@...radead.org
Subject: Re: [PATCH 0/1] scsi: qla1280: Make 64-bit DMA addressing a Kconfig option

On Thu, Jul 31, 2025 at 11:49 AM Maciej W. Rozycki <macro@...am.me.uk> wrote:
ng a simple way
> > to limit DMA addressing to 32-bits is relevant.
>
>  Given the description it seems to me it will best be handled as a quirk
> in arch/alpha/kernel/pci.c, at least in the interim.
>
>  If it turns out a generic issue with DAC handling in the Tsunami chipset,
> then a better approach would be a generic workaround for all potentially
> affected devices, but it does not appear we have existing infrastructure
> for that.  Just setting the global DMA mask would unnecessarily cripple
> 64-bit option cards as well, but it seems to me there might be something
> relevant in arch/mips/pci/fixup-sb1250.c; see `quirk_sb1250_pci_dac' and
> the comments above it.
>
>  The situation is a bit different here as the bus is a proper 64-bit one,
> but the quirk could only limit the individual DMA mask to 32 bits for
> devices that have no 64-bit memory BARs.  I suspect there are no proper
> 64-bit PCI option cards that only have I/O bars and I don't think there's
> any explicit status bit to tell 32-bit and 64-bit option cards apart.
>

This approach would probably also work. This would mean limiting
32-bit PCI cards from using DAC/monster window DMA on Tsunami based
Alphas. In practice, I believe that there are very few 32-bit PCI cards that
are likely to be used on Alphas that have drivers which support 64-bit DMA
addressing. The only example I've found so far is the qla1280 driver.

Since not all tsunami based Alphas support more than 2GB RAM there are
even fewer examples where this will be a real problem. I'm not 100% sure
that there is a generic issue with DAC handling in the Tsunami chipset but
I'm suspecting that there is. I think that for DS20/ES40 systems using
Qlogic ISP1040 SCSI controllers would be the most likely (maybe the only)
real use case that I can think of.

I can put together a v2 patch based on  a similar approach as
`quirk_sb1250_pci_dac' mentioned above, if this is a better approach
than limiting the DMA mask on only the qlogic driver (by using a module option)

Regards

Magnus