linux-kernel - Re: [PATCH] firewire: fw-ohci: sync AT dma buffer before use

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Message-Id: <1205361349.19600.24.camel@aries.csail.mit.edu>
Date:	Wed, 12 Mar 2008 18:35:49 -0400
From:	David Moore <dcm@....EDU>
To:	Jarod Wilson <jwilson@...hat.com>
Cc:	linux1394-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] firewire: fw-ohci: sync AT dma buffer before use

On Wed, 2008-03-12 at 17:43 -0400, Jarod Wilson wrote:
> diff --git a/drivers/firewire/fw-ohci.c b/drivers/firewire/fw-ohci.c
> index 081a434..fc45868 100644
> --- a/drivers/firewire/fw-ohci.c
> +++ b/drivers/firewire/fw-ohci.c
> @@ -780,6 +780,10 @@ at_context_queue_packet(struct context *ctx, struct fw_packet *packet)
>  
>  	context_append(ctx, d, z, 4 - z);
>  
> +	/* Sync the DMA buffer up for the device to read from */
> +	dma_sync_single_for_device(ohci->card.device, payload_bus,
> +				   packet->payload_length, DMA_TO_DEVICE);
> +
>  	/* If the context isn't already running, start it up. */
>  	reg = reg_read(ctx->ohci, CONTROL_SET(ctx->regs));
>  	if ((reg & CONTEXT_RUN) == 0)
> 

Two things:

1. The dma_sync should probably be done before the context_append
because the controller could in theory start reading the data as soon as
context_append is called right?

2. As an optimization, we should attempt to allocate the payload in the
lower 32-bits of physical memory, to prevent extra memcopies on x86_64.
I think this can be done by adding GFP_DMA32 to kmalloc where the
payload was allocated in fw-cdev.c.  There might be other places where
we would benefit from GFP_DMA32 also.  Of course, these optimizations
are probably better saved for another patch.

-David

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