linux-kernel - Re: [PATCH] staging: vc04_services: setup DMA and coherent mask

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Date:   Mon, 31 Oct 2016 11:40:58 -0700
From:   Michael Zoran <mzoran@...wfest.net>
To:     Eric Anholt <eric@...olt.net>, gregkh@...uxfoundation.org
Cc:     swarren@...dotorg.org, lee@...nel.org, daniels@...labora.com,
        noralf@...nnes.org, popcornmix@...il.com,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: vc04_services: setup DMA and coherent mask

On Mon, 2016-10-31 at 11:36 -0700, Eric Anholt wrote:
> Michael Zoran <mzoran@...wfest.net> writes:
> 
> > Setting the DMA mask is optional on 32 bit but
> > is mandatory on 64 bit.  Set the DMA mask and coherent
> > to force all DMA to be in the 32 bit address space.
> > 
> > This is considered a "good practice" and most drivers
> > already do this.
> > 
> > Signed-off-by: Michael Zoran <mzoran@...wfest.net>
> > ---
> >  .../staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c |
> > 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git
> > a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.
> > c
> > b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.
> > c
> > index a5afcc5..6fa2b5a 100644
> > ---
> > a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.
> > c
> > +++
> > b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.
> > c
> > @@ -97,6 +97,16 @@ int vchiq_platform_init(struct platform_device
> > *pdev, VCHIQ_STATE_T *state)
> >  	int slot_mem_size, frag_mem_size;
> >  	int err, irq, i;
> >  
> > +	/*
> > +	 * Setting the DMA mask is necessary in the 64 bit
> > environment.
> > +	 * It isn't necessary in a 32 bit environment but is
> > considered
> > +	 * a good practice.
> > +	 */
> > +	err = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
> 
> I think a better comment here would be simply:
> 
> /* VCHI messages between the CPU and firmware use 32-bit bus
> addresses. */
> 
> explaining why the value is chosen (once you know that the 32 bit
> restriction exists, reporting it is obviously needed).  I'm curious,
> though: what failed when you didn't set it?
> 

The comment is easy to change.

I don't have the log available ATM, but if I remember the DMA API's
bugcheck the first time that are used.  

I think this was a policy decision or something because the information
should be available in the dma-ranges.

If it's important, I can setup a test again without the change and e-
mail the logs.

If you look at the DWC2 driver you will see that it also sets this
mask.