netdev - Re: [PATCH] [v6] net: emac: emac gigabit ethernet controller driver

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Date:	Wed, 29 Jun 2016 17:34:57 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Timur Tabi <timur@...eaurora.org>
Cc:	netdev@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, sdharia@...eaurora.org,
	shankerd@...eaurora.org, vikrams@...eaurora.org,
	cov@...eaurora.org, gavidov@...eaurora.org, robh+dt@...nel.org,
	andrew@...n.ch, bjorn.andersson@...aro.org, mlangsdo@...hat.com,
	jcm@...hat.com, agross@...eaurora.org, davem@...emloft.net,
	f.fainelli@...il.com, catalin.marinas@....com
Subject: Re: [PATCH] [v6] net: emac: emac gigabit ethernet controller driver

On Wednesday, June 29, 2016 10:10:59 AM CEST Timur Tabi wrote:
> Arnd Bergmann wrote:
> > That's also not how it works: each device starts out with a 32-bit mask,
> > because that's what historically all PCI devices can do. If a device
> > is 64-bit DMA capable, it can extend the mask by passing DMA_BIT_MASK(64)
> > (or whatever it can support), and the platform code checks if that's
> > possible.
> 
> So if it's not possible, then dma_set_mask returns an error, and the 
> driver should try a smaller mask?  Doesn't that mean that every driver 
> for a 64-bit device should do this:
> 
>         for (i = 64; i >=32; i--) {
>                 ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(i));
>                 if (!ret)
>                         break;
>         }
> 
>         if (ret)
>                 return ret;
> 
> Sure, this is overkill, but it seems to me that the driver does not 
> really know what mask is actually valid, so it has to find the largest 
> mask that works.
> 

Usually drivers try 64-bit mask and 32-bit masks, and the 32 bit
mask is practically guaranteed to succeed.

Platforms will also allow allow the driver to set a mask that
is larger than what the bus supports, as long as all RAM is
reachable by the bus.

	Arnd