From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751448AbdFFKhh (ORCPT ); Tue, 6 Jun 2017 06:37:37 -0400 Received: from mail-it0-f42.google.com ([209.85.214.42]:37980 "EHLO mail-it0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751384AbdFFKhf (ORCPT ); Tue, 6 Jun 2017 06:37:35 -0400 MIME-Version: 1.0 In-Reply-To: <20170606102447.cftuyhtx7zo2fe2g@ninjato> References: <20170606092034.1516-1-wsa+renesas@sang-engineering.com> <20170606092034.1516-3-wsa+renesas@sang-engineering.com> <20170606102447.cftuyhtx7zo2fe2g@ninjato> From: Geert Uytterhoeven Date: Tue, 6 Jun 2017 12:37:33 +0200 X-Google-Sender-Auth: -tpsSrlgUEMwHITAZFsVHzYEzpA Message-ID: Subject: Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling To: Wolfram Sang Cc: Wolfram Sang , Linux I2C , Linux-Renesas , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Wolfram, On Tue, Jun 6, 2017 at 12:24 PM, Wolfram Sang wrote: >> > +Therefore, it is *not* mandatory that the buffer of an i2c message is DMA safe. >> > +It does not seem reasonable to apply additional burdens when the feature is so >> > +rarely used. However, it is recommended to use a DMA-safe buffer, if your >> > +message size is likely applicable for DMA (FIXME: > 8 byte?). >> >> So you expect drivers to fall back to PIO automatically if the buffer is >> not DMA safe. Sounds good to me. > > Yes, I strongly recommend that. Otherwise, drivers can always deal with > bounce buffers on their own. > >> However, your check for a DMA-capable buffer is invoked only if >> CONFIG_DMA_API_DEBUG is enabled: > > is *NOT* enabled! Oops ;-) >> #if !defined(CONFIG_DMA_API_DEBUG) >> if (!virt_addr_valid(msg->buf) || object_is_on_stack(msg->buf)) { >> pr_debug("msg buffer to 0x%04x might not be DMA capable\n", >> msg->addr); >> return -EFAULT; >> } >> #endif >> > > The #if block is there because DMA_API_DEBUG does a lot more, but if the > check in the helper is enabled, the core will fall back to PIO and you > won't get the additional info from DMA_API_DEBUG. > > I think this needs a comment :) > > Now OK? So it won't fall back to PIO if CONFIG_DMA_API_DEBUG is enabled? Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds