linux-kernel - Re: [PATCH 0/3] mtd: spi-nor: fix DMA-unsafe buffer issue between MTD and SPI

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Date:   Tue, 26 Dec 2017 18:45:28 +0000
From:   Trent Piepho <tpiepho@...inj.com>
To:     "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "cyrille.pitchen@...ev4u.fr" <cyrille.pitchen@...ev4u.fr>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "computersforpeace@...il.com" <computersforpeace@...il.com>,
        "boris.brezillon@...e-electrons.com" 
        <boris.brezillon@...e-electrons.com>,
        "vigneshr@...com" <vigneshr@...com>,
        "richard@....at" <richard@....at>,
        "marek.vasut@...il.com" <marek.vasut@...il.com>
CC:     "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "nicolas.ferre@...rochip.com" <nicolas.ferre@...rochip.com>,
        "robh@...nel.org" <robh@...nel.org>,
        "radu.pirea@...rochip.com" <radu.pirea@...rochip.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 0/3] mtd: spi-nor: fix DMA-unsafe buffer issue between MTD
 and SPI

On Sun, 2017-12-24 at 05:36 +0100, Cyrille Pitchen wrote:
> this series tries to solve a long time issue of compatibility between the
> MTD and SPI sub-systems about whether we should use DMA-safe memory.

Can this should replace SoC specific fixes like:

c687c46e9e45
spi: spi-ti-qspi: Use bounce buffer if read buffer is not DMA'ble

7094576ccdc3
spi: atmel: fix corrupted data issue on SAM9 family SoCs

Or, since this only fixes instances of DMA-unsafe buffers used in
access to SPI NOR flash chips, and since there are other SPI master
interface users, those chip specific fixes in some/all spi master
drivers are still needed to fix transfers not originated via spi-nor? 
Or are all the previous fixes necessary because of spi-nor flash (via
ubifs or jffs2) and once that's fixed via this series there are no more
originators of dma-unsafe buffers?

> The SPI sub-system has already implemented a work-around on its side,
> based on the spi_map_buf() function. However this function has its own
> limitation too. Especially, even if it builds a 'struct scatterlist' from
> a vmalloc'ed buffer, calling dma_map_sg() is still not safe on all
> architectures. Especially, on ARM cores using either VIPT or VIVT data
> caches, dma_map_sg() doesn't take the cache aliases issue into account.
> Then numerous crashes were reported for such architectures.

Could the above issue also be fixed via calls to
flush_kernel_vmap_range() and invalidate_kernel_vmap_range() to keep
the data cache valid?  Or is there a reason that won't work?