linux-kernel - Re: [PATCH v2 22/27] thunderbolt: Add support for DMA configuration based mailbox

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Date:   Sun, 28 May 2017 12:10:47 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <yehezkel.bernat@...el.com>,
        Lukas Wunner <lukas@...ner.de>,
        Amir Levy <amir.jer.levy@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Mario Limonciello <Mario.Limonciello@...l.com>,
        Jared.Dominguez@...l.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 22/27] thunderbolt: Add support for DMA configuration
 based mailbox

On Sat, May 27, 2017 at 07:08:34PM +0300, Andy Shevchenko wrote:
> On Fri, May 26, 2017 at 7:09 PM, Mika Westerberg
> <mika.westerberg@...ux.intel.com> wrote:
> > The DMA (NHI) port of a switch provides access to the NVM of the host
> > controller (and devices starting from Intel Alpine Ridge). The NVM
> > contains also more complete DROM for the root switch including vendor
> > and device identification strings.
> 
> > +               ret = dma_port_flash_read_block(dma, address, dma->buf,
> > +                                               ALIGN(nbytes, 4));
> > +               if (ret) {
> > +                       if (ret == -ETIMEDOUT) {
> > +                               if (retries--)
> > +                                       continue;
> > +                               ret = -EIO;
> > +                       }
> > +                       return ret;
> > +               }
> > +
> > +               memcpy(buf, dma->buf + offset, nbytes);
> 
> > +       do {
> > +               u32 nbytes = min_t(u32, size, MAIL_DATA_DWORDS * 4);
> > +               int ret;
> > +
> > +               memcpy(dma->buf + offset, buf, nbytes);
> > +
> > +               ret = dma_port_flash_write_block(dma, address, buf, nbytes);
> > +               if (ret) {
> > +                       if (ret == -ETIMEDOUT) {
> > +                               if (retries--)
> > +                                       continue;
> > +                               ret = -EIO;
> > +                       }
> > +                       return ret;
> > +               }
> 
> Just to be sure I didn't miss anything.
> Can't we just map buffer into DMA capable address space instead of memcpy()'ing?

The buffer is there to handle unaligned (non-dword) reads and writes. I
suppose you could do that but it adds more complexity than it is worth
IMHO because this is about sending and receiving messages over low-speed
control channel.

If there is a need to optimize things like this, I think we can do that
later on.