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[82.0.78.162]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a35ca8874bsm25088187f8f.67.2025.05.22.18.13.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 May 2025 18:13:09 -0700 (PDT) Message-ID: <54f84504-b40b-40a1-ac3e-b6baeec8ec0d@gmail.com> Date: Fri, 23 May 2025 02:13:07 +0100 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v16 22/22] sfc: support pio mapping based on cxl To: Dan Williams , alejandro.lucero-palau@amd.com, linux-cxl@vger.kernel.org, netdev@vger.kernel.org, edward.cree@amd.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, edumazet@google.com, dave.jiang@intel.com Cc: Alejandro Lucero , Jonathan Cameron References: <20250514132743.523469-1-alejandro.lucero-palau@amd.com> <20250514132743.523469-23-alejandro.lucero-palau@amd.com> <682e4a2c481d6_1626e1008e@dwillia2-xfh.jf.intel.com.notmuch> Content-Language: en-GB From: Edward Cree In-Reply-To: <682e4a2c481d6_1626e1008e@dwillia2-xfh.jf.intel.com.notmuch> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 21/05/2025 22:48, Dan Williams wrote: > Maybe that would be more obvious to me if I knew what a "PIO buffer" was > used for currently, but some more words about the why of all this would > help clarify if the design is making the right complexity vs benefit > tradeoffs. A PIO buffer is a region of device memory to which the driver can write packet data for TX, so that when the device handles the transmit doorbell it doesn't have to DMA that data across from host memory. Essentially it's spending CPU time to save a round-trip across the PCIe bus, reducing latency; the driver heuristically decides whether a TX is more bandwidth- or latency-sensitive, and in the latter case uses PIO. I don't know too much about the CXL side of things (hopefully Alejandro will elaborate) but AIUI using CXL instead of PCIe for this reduces the latency further. Some of the above information should probably be added to the series cover letter or this patch description.