linux-kernel - Re: [PATCH v9 6/9] i3c: master: Add driver for Cadence IP

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Date:   Thu, 25 Oct 2018 00:52:39 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     Wolfram Sang <wsa@...-dreams.de>, <linux-i2c@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>, <linux-doc@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Przemyslaw Sroka <psroka@...ence.com>,
        Arkadiusz Golec <agolec@...ence.com>,
        Alan Douglas <adouglas@...ence.com>,
        Bartosz Folta <bfolta@...ence.com>,
        Damian Kos <dkos@...ence.com>,
        Alicja Jurasik-Urbaniak <alicja@...ence.com>,
        Cyprian Wronka <cwronka@...ence.com>,
        Suresh Punnoose <sureshp@...ence.com>,
        Rafal Ciepiela <rafalc@...ence.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Nishanth Menon <nm@...com>, Rob Herring <robh+dt@...nel.org>,
        Pawel Moll <pawel.moll@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ian Campbell <ijc+devicetree@...lion.org.uk>,
        Kumar Gala <galak@...eaurora.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Vitor Soares <Vitor.Soares@...opsys.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Xiang Lin <Xiang.Lin@...aptics.com>,
        <linux-gpio@...r.kernel.org>, Sekhar Nori <nsekhar@...com>,
        Przemyslaw Gaj <pgaj@...ence.com>,
        Peter Rosin <peda@...ntia.se>,
        Mike Shettel <mshettel@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v9 6/9] i3c: master: Add driver for Cadence IP

On Wed, 24 Oct 2018 17:43:00 -0500
Grygorii Strashko <grygorii.strashko@...com> wrote:

> On 10/24/18 4:04 PM, Boris Brezillon wrote:
> > On Wed, 24 Oct 2018 15:25:17 -0500
> > Grygorii Strashko <grygorii.strashko@...com> wrote:
> >   
> >> On 10/24/18 1:20 PM, Boris Brezillon wrote:  
> >>> Hi Arnd,
> >>>
> >>> On Mon, 22 Oct 2018 15:34:01 +0200
> >>> Boris Brezillon <boris.brezillon@...tlin.com> wrote:
> >>>
> >>>      
> >>>> +
> >>>> +static void cdns_i3c_master_rd_from_rx_fifo(struct cdns_i3c_master *master,
> >>>> +					    u8 *bytes, int nbytes)
> >>>> +{
> >>>> +	readsl(master->regs + RX_FIFO, bytes, nbytes / 4);  
> >>>
> >>> Vitor reported a problem with readsl(): this function expects the 2nd
> >>> argument to be aligned on 32-bit, which is not guaranteed here. Unless
> >>> you see a better solution, I'll switch back to a loop doing:
> >>>
> >>> 	for (i = 0; i < nbytes; i += 4) {
> >>> 		u32 tmp = __raw_readl(...);  
> >>
> >> Pls, do not use __raw io.  
> > 
> > Except this is exactly what I want here, unless you have a
> > replacement for "readl() without a mem-barrier and without endianness
> > conversion"
> >   
> 
> Not sure why endianness is the problem. readl_relaxed?

Because we want to read a stream of bytes, and, if we have a CPU that is
operating in big-endian (ARM kernels can configured in BE or LE), byte
ordering will be messed up (the controller is LE).

If I use readl_relaxed(), I'll then have to call cpu_to_le32(), and
finally copy the result to the buffer.

> Sry, I've missed that this is part of the driver not i3c core,
> so minor/ignore.
>