linux-kernel - Re: [patch 6/7] SDHCI: Check DMA for overruns at end of transfer

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Message-ID: <20081104012840.GA16742@khazad-dum.debian.net>
Date:	Mon, 3 Nov 2008 23:28:40 -0200
From:	Henrique de Moraes Holschuh <hmh@....eng.br>
To:	Ben Dooks <ben-linux@...ff.org>
Cc:	linux-kernel@...r.kernel.org, drzeus-mmc@...eus.cx,
	sdhci-devel@...t.drzeus.cx
Subject: Re: [patch 6/7] SDHCI: Check DMA for overruns at end of transfer

On Mon, 03 Nov 2008, Ben Dooks wrote:
> On Mon, Nov 03, 2008 at 07:12:00PM -0200, Henrique de Moraes Holschuh wrote:
> > Maybe I didn't understand it right, but if the DMA controller could overrun
> > a buffer, don't you ALSO need to add defensive padding (i.e. increase the
> > buffer) to make sure nothing important gets overrun?
> 
> This is only generated by problems elsewhere in the driver, such as
> getting the timeout clock wrong. It is here just as a precaution and
> as an aide to debugging, it should not trigger in normal circumstances.

Then why is it just a WARN_ON, since you had a rogue DMA operation
overwriting unknown kernel memory?  Seems like an outright BUG_ON to me.

> There is a seperate problem where the DMA buffer is passed from the stack
> which is, IIRC, a complete no-no under Linux. 

Can't say much on that.  I just found it strange that something as damaging
as an overrun was only getting a WARN_ON and no defensive measure.  If it is
not going to happen normally, it might not require a defensive buffer, but
once it happens, it looks like one must reboot ASAP from what you said...

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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