linux-kernel - Re: [PATCH v5 3/5] drivers/soc/litex: add LiteX SoC Controller driver

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Date:   Wed, 29 Apr 2020 13:21:11 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Mateusz Holenko <mholenko@...micro.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, devicetree@...r.kernel.org,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>
Cc:     Stafford Horne <shorne@...il.com>,
        Karol Gugala <kgugala@...micro.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Filip Kokosinski <fkokosinski@...micro.com>,
        Pawel Czarnecki <pczarnecki@...ernships.antmicro.com>,
        Joel Stanley <joel@....id.au>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Maxime Ripard <mripard@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Sam Ravnborg <sam@...nborg.org>,
        Icenowy Zheng <icenowy@...c.io>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        linux-kernel@...r.kernel.org, "Gabriel L. Somlo" <gsomlo@...il.com>
Subject: Re: [PATCH v5 3/5] drivers/soc/litex: add LiteX SoC Controller
 driver

On Mon, 2020-04-27 at 11:13 +0200, Mateusz Holenko wrote:
> As Gabriel Somlo <gsomlo@...il.com> suggested to me, I could still use
> readl/writel/ioread/iowrite() standard functions providing memory
> barriers *and* have values in CPU native endianness by using the
> following constructs:
> 
> `le32_to_cpu(readl(addr))`
> 
> and
> 
> `writel(cpu_to_le32(value), addr)`
> 
> as le32_to_cpu/cpu_to_le32():
> - does nothing on LE CPUs and
> - reorders bytes on BE CPUs which in turn reverts swapping made by
> readl() resulting in returning the original value.

It's a bit sad... I don't understand why you need this. The HW has a
fied endian has you have mentioned earlier (and that is a good design).

The fact that you are trying to shove things into a "smaller pipe" than
the actual register shouldn't affect at what address the MSB and LSB
reside. And readl/writel (or ioread32/iowrite32) will always be LE as
well, so will match the HW layout. Thus I don't see why you need to
play swapping games here.

This however would be avoided completely if the HW was a tiny bit
smarter and would do the multi-beat access for you which shouldn't be
terribly hard to implement.

That said, it would be even clearer if you just open coded the 2 or 3
useful cases: 32/8, 32/16 and 32/32. The loop with calculated shifts
(and no masks) makes the code hard to understand.

Cheers,
Ben.