linux-kernel - Re: [RESEND2 PATCH 1/3] memremap: add MEMREMAP

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Date:	Mon, 8 Feb 2016 12:03:17 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Brian Starkey <brian.starkey@....com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND2 PATCH 1/3] memremap: add MEMREMAP_WC flag

On Mon,  8 Feb 2016 17:30:50 +0000 Brian Starkey <brian.starkey@....com> wrote:

> Add a flag to memremap() for writecombine mappings. Mappings satisfied
> by this flag will not be cached, however writes may be delayed or
> combined into more efficient bursts. This is most suitable for
> buffers written sequentially by the CPU for use by other DMA devices.
> 
> ...

The patch generally looks OK to me.  It generates rejects against
linux-next because of some janitorial changes in memremap.c.


> @@ -101,6 +107,11 @@ void *memremap(resource_size_t offset, size_t size, unsigned long flags)
>  		addr = ioremap_wt(offset, size);
>  	}
>  
> +	if (!addr && (flags & MEMREMAP_WC)) {
> +		flags &= ~MEMREMAP_WC;
> +		addr = ioremap_wc(offset, size);
> +	}
> +
>  	return addr;
>  }

The modifications of `flags' is unneeded (and the compiler will remove
it).  And generally the modification of incoming args is a bit nasty
IMO - I find it's better to treat them as const - part of the calling
environment which can be relied upon to be unaltered as the code
evolves.