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From: Arnd Bergmann <arnd@arndb.de>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	Lada Trimasova <Lada.Trimasova@synopsys.com>,
	Alexey Brodkin <Alexey.Brodkin@synopsys.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Noam Camus <noamc@ezchip.com>,
	"linux-snps-arc@lists.infradead.org"
	<linux-snps-arc@lists.infradead.org>
Subject: Re: [PATCH] arc: use little endian accesses
Date: Thu, 10 Mar 2016 08:45:30 +0100
Message-ID: <201603100845.30602.arnd@arndb.de> (raw)
In-Reply-To: <C2D7FE5348E1B147BCA15975FBA23075F4E8EDF9@us01wembx1.internal.synopsys.com>

On Thursday 10 March 2016, Vineet Gupta wrote:
> On Wednesday 09 March 2016 10:51 PM, Lada Trimasova wrote:
> > Memory access primitives should use cpu_to_le16, cpu_to_le32, le16_to_cpu
> > and le32_to_cpu because it is not really guaranteed that drivers handles
> > any ordering themselves.
> 
> That is the driver issue. readxx as API simply returns data in native endianness.
> We've had EZChip running big endian and so far and they didn't need this change.

Most drivers using readl() or readl_relaxed() expect those to perform byte
swaps on big-endian architectures, as the registers tend to be fixed endian,
so the change seems reasonable.

It depends a little bit on how endian mode is implemented in the CPU: do you
follow the normal model of swapping byte order in the load/store instructions
of the CPU when running big-endian, or is the CPU always running little-endian
but the bus addresses get mangled on byte accesses to give the illusion
of a big-endian system?

	Arnd

From: Arnd Bergmann <arnd@arndb.de>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: Lada Trimasova <Lada.Trimasova@synopsys.com>,
	"linux-snps-arc@lists.infradead.org"
	<linux-snps-arc@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	Alexey Brodkin <Alexey.Brodkin@synopsys.com>,
	Noam Camus <noamc@ezchip.com>
Subject: Re: [PATCH] arc: use little endian accesses
Date: Thu, 10 Mar 2016 08:45:30 +0100
Message-ID: <201603100845.30602.arnd@arndb.de> (raw)
Message-ID: <20160310074530.k0q3Vq-jwJ3fz76rgpjyg0IyYEPcoqw0ufsSRYDeMQQ@z> (raw)
In-Reply-To: <C2D7FE5348E1B147BCA15975FBA23075F4E8EDF9@us01wembx1.internal.synopsys.com>

On Thursday 10 March 2016, Vineet Gupta wrote:
> On Wednesday 09 March 2016 10:51 PM, Lada Trimasova wrote:
> > Memory access primitives should use cpu_to_le16, cpu_to_le32, le16_to_cpu
> > and le32_to_cpu because it is not really guaranteed that drivers handles
> > any ordering themselves.
> 
> That is the driver issue. readxx as API simply returns data in native endianness.
> We've had EZChip running big endian and so far and they didn't need this change.

Most drivers using readl() or readl_relaxed() expect those to perform byte
swaps on big-endian architectures, as the registers tend to be fixed endian,
so the change seems reasonable.

It depends a little bit on how endian mode is implemented in the CPU: do you
follow the normal model of swapping byte order in the load/store instructions
of the CPU when running big-endian, or is the CPU always running little-endian
but the bus addresses get mangled on byte accesses to give the illusion
of a big-endian system?

	Arnd

  parent reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-09 17:21 Lada Trimasova
2016-03-10  5:05 ` Vineet Gupta
2016-03-10  5:19   ` Vineet Gupta
2016-03-10  5:19     ` Vineet Gupta
2016-03-10  7:44   ` Alexey Brodkin
2016-03-10  9:55     ` Vineet Gupta
2016-03-10 18:57       ` Lada Trimasova
2016-03-10 18:57         ` Lada Trimasova
2016-03-10 19:23         ` Arnd Bergmann
2016-03-11 12:44           ` Vineet Gupta
2016-03-12  4:20             ` Vineet Gupta
2016-03-12  4:20               ` Vineet Gupta
2016-03-11  5:07         ` Noam Camus
2016-03-11  5:07           ` Noam Camus
2016-03-10  7:45   ` Arnd Bergmann [this message]
2016-03-10  7:45     ` Arnd Bergmann
2016-03-11  5:18     ` Vineet Gupta

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