linux-kernel - Re: [PATCH] drivers: rtl818x: request DMA-able memory

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Message-ID: <1288016584.3716.2.camel@jlt3.sipsolutions.net>
Date:	Mon, 25 Oct 2010 16:23:04 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Hin-Tak Leung <htl10@...rs.sourceforge.net>
Cc:	Larry Finger <Larry.Finger@...inger.net>,
	Serafeim Zanikolas <sez@...ian.org>, herton@...driva.com.br,
	linville@...driver.com, joe@...ches.com, davem@...emloft.net,
	linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers: rtl818x: request DMA-able memory

On Mon, 2010-10-25 at 15:11 +0100, Hin-Tak Leung wrote:

> >> Despite the indicated intention in comment, the kmalloc() call was not
> >> explicitly requesting memory from ZONE_DMA.

> I had a quick look for similiar constructs and AFAIK only the b43/b43legacy 
> drivers uses DMA buffers. Seems to be a rare practice. Is that something we 
> should or should not do?

I think there's some confusion here about ZONE_DMA vs. DMA-able memory.
All memory you get with kmalloc can be used for DMA, GFP_DMA means using
ZONE_DMA which is a hack for ISA (and in b43 maybe PCMCIA/Cardbus)
devices to put memory into something they can address. I don't think the
latter is necessary for USB devices.

johannes

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