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From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Salter <msalter@redhat.com>
Cc: ming.lei@canonical.com, stern@rowland.harvard.edu,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/3] define ARM-specific dma_coherent_write_sync
Date: Tue, 6 Sep 2011 15:48:51 +0100	[thread overview]
Message-ID: <CAHkRjk6TD4Qg-b3nkCE_wwj-tWr-_5evndxo3f+_6gtXEV6gNg@mail.gmail.com> (raw)
In-Reply-To: <1315319837.2313.1.camel@deneb.redhat.com>

On 6 September 2011 15:37, Mark Salter <msalter@redhat.com> wrote:
> On Tue, 2011-09-06 at 15:32 +0100, Catalin Marinas wrote:
>> That's what mb() and wmb() do already, at least on ARM. Why do we need
>> another API? IIRC from past discussions on linux-arch around barriers,
>> the mb() should be sufficient in the case of DMA coherent buffers.
>> That's why macros like writel() on ARM have the mb() added by default
>> (for cases where you start the DMA transfer by writing to a device
>> register).
>
> For USB EHCI, the driver does not necessarily write to a register after
> writing to DMA coherent memory. In some cases, the controller polls for
> information written by the driver.

So as I understand, you would like to force the eviction from the
write buffer rather than waiting for it to be drained. On ARM, the
write buffer is eventually flushed, so there is no strict timing
guarantee. It could take longer if the processor immediately starts
polling some memory location for example, but in this case a simple
barrier would do.

-- 
Catalin

WARNING: multiple messages have this Message-ID (diff)
From: catalin.marinas@arm.com (Catalin Marinas)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH 2/3] define ARM-specific dma_coherent_write_sync
Date: Tue, 6 Sep 2011 15:48:51 +0100	[thread overview]
Message-ID: <CAHkRjk6TD4Qg-b3nkCE_wwj-tWr-_5evndxo3f+_6gtXEV6gNg@mail.gmail.com> (raw)
In-Reply-To: <1315319837.2313.1.camel@deneb.redhat.com>

On 6 September 2011 15:37, Mark Salter <msalter@redhat.com> wrote:
> On Tue, 2011-09-06 at 15:32 +0100, Catalin Marinas wrote:
>> That's what mb() and wmb() do already, at least on ARM. Why do we need
>> another API? IIRC from past discussions on linux-arch around barriers,
>> the mb() should be sufficient in the case of DMA coherent buffers.
>> That's why macros like writel() on ARM have the mb() added by default
>> (for cases where you start the DMA transfer by writing to a device
>> register).
>
> For USB EHCI, the driver does not necessarily write to a register after
> writing to DMA coherent memory. In some cases, the controller polls for
> information written by the driver.

So as I understand, you would like to force the eviction from the
write buffer rather than waiting for it to be drained. On ARM, the
write buffer is eventually flushed, so there is no strict timing
guarantee. It could take longer if the processor immediately starts
polling some memory location for example, but in this case a simple
barrier would do.

-- 
Catalin

  reply	other threads:[~2011-09-06 14:49 UTC|newest]

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-08-31 21:30 [PATCH 0/3] RFC: addition to DMA API Mark Salter
2011-08-31 21:30 ` Mark Salter
2011-08-31 21:30 ` [PATCH 1/3] add dma_coherent_write_sync " Mark Salter
2011-08-31 21:30   ` Mark Salter
2011-09-01  2:59   ` Josh Cartwright
2011-09-01  2:59     ` Josh Cartwright
2011-09-01  9:57   ` Michał Mirosław
2011-09-01  9:57     ` Michał Mirosław
2011-09-01 12:36     ` Mark Salter
2011-09-01 12:36       ` Mark Salter
2011-09-06 14:30       ` Catalin Marinas
2011-09-06 14:30         ` Catalin Marinas
2011-08-31 21:30 ` [PATCH 2/3] define ARM-specific dma_coherent_write_sync Mark Salter
2011-08-31 21:30   ` Mark Salter
2011-09-06 14:32   ` Catalin Marinas
2011-09-06 14:32     ` Catalin Marinas
2011-09-06 14:37     ` Mark Salter
2011-09-06 14:37       ` Mark Salter
2011-09-06 14:48       ` Catalin Marinas [this message]
2011-09-06 14:48         ` Catalin Marinas
2011-09-06 15:02         ` Mark Salter
2011-09-06 15:02           ` Mark Salter
2011-10-03  1:40           ` Jon Masters
2011-10-03  1:40             ` Jon Masters
2011-10-03  8:44             ` Catalin Marinas
2011-10-03  8:44               ` Catalin Marinas
2011-10-03  9:24               ` Jon Masters
2011-10-03  9:24                 ` Jon Masters
2011-08-31 21:30 ` [PATCH 3/3] add dma_coherent_write_sync calls to USB EHCI driver Mark Salter
2011-08-31 21:30   ` Mark Salter
2011-09-01  2:33   ` Ming Lei
2011-09-01  2:33     ` Ming Lei
2011-09-01  2:09 ` [PATCH 0/3] RFC: addition to DMA API Ming Lei
2011-09-01  2:09   ` Ming Lei
2011-09-01  3:09   ` Alan Stern
2011-09-01  3:09     ` Alan Stern
2011-09-01  3:41     ` Ming Lei
2011-09-01  3:41       ` Ming Lei
2011-09-01  8:45       ` Will Deacon
2011-09-01  8:45         ` Will Deacon
2011-09-01  9:14         ` Ming Lei
2011-09-01  9:14           ` Ming Lei
2011-09-01 15:42           ` Alan Stern
2011-09-01 15:42             ` Alan Stern
2011-09-01 16:04             ` Russell King - ARM Linux
2011-09-01 16:04               ` Russell King - ARM Linux
2011-09-01 17:31               ` Will Deacon
2011-09-01 17:31                 ` Will Deacon
2011-09-01 18:07                 ` Russell King - ARM Linux
2011-09-01 18:07                   ` Russell King - ARM Linux
2011-09-01 19:14                 ` Mark Salter
2011-09-01 19:14                   ` Mark Salter
2011-09-01 15:22       ` Alan Stern
2011-09-01 15:22         ` Alan Stern
2011-09-01 15:56         ` Ming Lei
2011-09-01 15:56           ` Ming Lei
2011-09-01 16:48           ` Alan Stern
2011-09-01 16:48             ` Alan Stern
2011-09-02  0:59             ` Ming Lei
2011-09-02  0:59               ` Ming Lei
2011-09-02 13:53               ` Alan Stern
2011-09-02 13:53                 ` Alan Stern
2011-09-01  9:11 ` Will Deacon
2011-09-01  9:11   ` Will Deacon

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