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Date:   Tue, 10 Sep 2019 08:13:48 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Cc:     Thierry Reding <thierry.reding@gmail.com>,
        Christoph Hellwig <hch@lst.de>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Jens Axboe <axboe@kernel.dk>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Simon Horman <horms+renesas@verge.net.au>,
        Jon Hunter <jonathanh@nvidia.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-mmc@vger.kernel.org" <linux-mmc@vger.kernel.org>,
        "linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>
Subject: Re: [PATCH 1/3] block: Respect the device's maximum segment size
Message-ID: <20190910061348.GA30982@lst.de>
References: <20190909125658.30559-1-thierry.reding@gmail.com> <20190909125658.30559-2-thierry.reding@gmail.com> <20190909161331.GA19650@lst.de> <20190909191911.GC23804@mithrandir> <TYAPR01MB454470364B682B9BF708E557D8B60@TYAPR01MB4544.jpnprd01.prod.outlook.com>
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On Tue, Sep 10, 2019 at 02:03:17AM +0000, Yoshihiro Shimoda wrote:
> I'm sorry for causing this trouble on your environment. I have a proposal to
> resolve this issue. The mmc_host struct will have a new caps2 flag
> like MMC_CAP2_MERGE_CAPABLE and add a condition into the queue.c like below.
> 
> +	if (host->caps2 & MMC_CAP2_MERGE_CAPABLE &&
> +	    host->max_segs < MMC_DMA_MAP_MERGE_SEGMENTS &&
> 	    dma_get_merge_boundary(mmc_dev(host)))
> 		host->can_dma_map_merge = 1;
> 	else
> 		host->can_dma_map_merge = 0;
> 
> After that, all mmc controllers disable the feature as default, and if a mmc
> controller has such capable, the host driver should set the flag.

That sounds sensible to me.  Alternatively we'd have to limit
max_sectors to 16-bit values for sdhci if using an iommu that can
merge.