linux-kernel - Re: [PATCH v4 1/5] dma-mapping: Add dma_opt_mapping

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Date:   Tue, 28 Jun 2022 12:27:38 +0100
From:   John Garry <john.garry@...wei.com>
To:     Robin Murphy <robin.murphy@....com>,
        <damien.lemoal@...nsource.wdc.com>, <joro@...tes.org>,
        <will@...nel.org>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <hch@....de>,
        <m.szyprowski@...sung.com>
CC:     <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-ide@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <iommu@...ts.linux.dev>, <linux-scsi@...r.kernel.org>,
        <linuxarm@...wei.com>
Subject: Re: [PATCH v4 1/5] dma-mapping: Add dma_opt_mapping_size()

On 28/06/2022 12:23, Robin Murphy wrote:
>> +
>> +    size_t
>> +    dma_opt_mapping_size(struct device *dev);
>> +
>> +Returns the maximum optimal size of a mapping for the device. Mapping 
>> large
>> +buffers may take longer so device drivers are advised to limit total DMA
>> +streaming mappings length to the returned value.
> 
> Nit: I'm not sure "advised" is necessarily the right thing to say in 
> general - that's only really true for a caller who cares about 
> throughput of churning through short-lived mappings more than anything 
> else, and doesn't take a significant hit overall from splitting up 
> larger requests. I do think it's good to clarify the exact context of 
> "optimal" here, but I'd prefer to be objectively clear that it's for 
> workloads where the up-front mapping overhead dominates.

Ok, sure, I can make that clear.

Thanks,
John