linux-kernel - Re: [PATCH v6 1/1] dmaengine: switchtec-dma: Introduce Switchtec DMA engine PCI driver

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZSaLoaenhsEG4/IP@matsya>
Date:   Wed, 11 Oct 2023 17:18:49 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Kelvin.Cao@...rochip.com
Cc:     dmaengine@...r.kernel.org, George.Ge@...rochip.com,
        linux-kernel@...r.kernel.org, logang@...tatee.com,
        christophe.jaillet@...adoo.fr, hch@...radead.org
Subject: Re: [PATCH v6 1/1] dmaengine: switchtec-dma: Introduce Switchtec DMA
 engine PCI driver

On 10-10-23, 21:23, Kelvin.Cao@...rochip.com wrote:
> On Mon, 2023-10-09 at 11:08 +0530, Vinod Koul wrote:

> > > u64 size_to_transfer;
> > 
> > Why cant the client driver write to doorbell, is there anything which
> > prevents us from doing so?
> 
> I think the potential challenge here for the client driver to ring db
> is that the client driver (host RC) is a different requester in the
> PCIe hierarchy compared to DMA EP, in which case PCIe ordering need to
> be considered. 
> 
> As PCIe ensures that reads don't pass writes, we can insert a read DMA
> operation with DMA_PREP_FENSE flag in between the two DMA writes (one
> for data transfer and one for notification) to ensure the ordering for
> the same requester DMA EP. I'm not sure if the RC could ensure the same
> ordering if the client driver issue MMIO write to db after the data DMA
> and read DMA completion, so that the consumer is guaranteed the
> transferred data is ready in memory when the db is triggered by the
> client MMIO write. I guess it's still doable with MMIO write but just
> some special consideration needed. 

Given that it is a single value, overhead of doing a new txn would be
higher than a mmio write! I think that should be preferred

-- 
~Vinod