From mboxrd@z Thu Jan  1 00:00:00 1970
From: mzoran@crowfest.net (Michael Zoran)
Date: Mon, 31 Oct 2016 11:40:58 -0700
Subject: [PATCH] staging: vc04_services: setup DMA and coherent mask
In-Reply-To: <87twbsqsb4.fsf@eliezer.anholt.net>
References: <20161028171159.23973-1-mzoran@crowfest.net>
 <87twbsqsb4.fsf@eliezer.anholt.net>
Message-ID: <1477939258.30971.1.camel@crowfest.net>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Mon, 2016-10-31 at 11:36 -0700, Eric Anholt wrote:
> Michael Zoran <mzoran@crowfest.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@crowfest.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.