linux-kernel - Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi

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Date:	Thu, 5 Sep 2013 20:54:40 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-pci@...r.kernel.org, linux-ide@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>, Joerg Roedel <joro@...tes.org>,
	Jan Beulich <JBeulich@...e.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface

On Thu, Sep 05, 2013 at 11:44:36AM -0400, Tejun Heo wrote:
> Given that multiple MSI is something which isn't too popular / already
> superseded and that the condition is highly unlikely, do we really
> care about possible partial success?  This sort of interface is
> unnecessarily complex and actively harmful.  It forces all users to
> wonder what could possibly happen and implement all sorts of nutty
> fallback logic which is highly likely to be error-prone on both the
> software and hardware side.  Seriously, how much testing would such
> code path would get both on the driver and firmware sides?
> 
> It's an operation which isn't too likely to fail with a firm
> known-to-work fallback.  It's pointless and error-prone to try to
> extract the last point zero zero one percent.

I assume reasons for having this type of interface at the moment of
taking design decision about pci_enable_msi_block() still hold true.
I do not know what those reasons were, but I think the fact multiple
MSIs are rarely used and MSI-X exists does not invalidate them now.

I did consider the other argument - since pci_enable_msi_block_part()
is explicitly provided with a value of MME the caller will not be
satisfied with any other value and hence a repeated call with a lesser
MME does not make sense for the caller. Therefore we could just fail
in case the architecture returned a positive value. Same result, but
different reasoning.

At the moment I still prefer pci_enable_msi_block_part() to be similar
to pci_enable_msi_block(). I do agree the fallback logic is error-prone,
but I would not dare to scrap it all right away.

Bjorn?

> 
> Thanks.
> 
> -- 
> tejun

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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