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Wed, 07 Jul 2021 15:54:24 +0000 Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m19sR-00FFAl-PM; Wed, 07 Jul 2021 15:54:21 +0000 Received: by mail.kernel.org (Postfix) with ESMTPSA id 477B561CBF; Wed, 7 Jul 2021 15:54:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1625673259; bh=jBYB8E2kxgHKI+p6Ge17CrAqEwZGHZvxadKDfFVaIpg=; h=Date:From:To:Cc:Subject:In-Reply-To:From; b=i2oNik5do5AHOLInc4OUUnh1F5HFHYK0kr4sWV3miNytCJw7/lDxUrFNZc8zt+nVY OiD9V2ljFumrYj8FRbVYSSjxPfzFDRrGLSj6Wx3m5uQA2m8CZtbkBk/RKXZ+aWJb2E SmHiGI0lmJLrmjHxBW1okpUEq0lX4ITih1OyeYBigaiJ2xoKatvbZdQ2hBOYN8FPYD 10AdgnOw8vzYCVq5JVfO5sByQHdJvhvdYC2IBE/4TnEELQcKv8QoPSP7J9TBO3vLyg T+hSNeXjX71oyYS4rQ5JrGBRvKfteyyO2NwNv3nYVTt1O1mfPW6tTySCVYE1nJY/w3 rXj5dhh7qvz2Q== Date: Wed, 7 Jul 2021 10:54:18 -0500 From: Bjorn Helgaas To: Neil Armstrong Cc: Art Nikpal , Huacai Chen , =?utf-8?B?6ZmI5Y2O5omN?= , Yue Wang , Kevin Hilman , Lorenzo Pieralisi , Rob Herring , Krzysztof Wilczynski , Jerome Brunet , Christian Hewitt , Martin Blumenstingl , PCI , linux-arm-kernel , "open list:ARM/Amlogic Meson..." , LKML , Artem Lapkin , Nick Xie , Gouwa Wang Subject: Re: [PATCH 0/4] PCI: replace dublicated MRRS limit quirks Message-ID: <20210707155418.GA897940@bjorn-Precision-5520> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <1271fa28-dddd-01a3-5ad5-e3b4898f5482@baylibre.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210707_085419_903038_640B30F8 X-CRM114-Status: GOOD ( 19.23 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jul 06, 2021 at 11:54:05AM +0200, Neil Armstrong wrote: > In their Designware PCIe controller driver, amlogic sets the > Max_Payload_Size & Max_Read_Request_Size to 256: > https://elixir.bootlin.com/linux/latest/source/drivers/pci/controller/dwc/pci-meson.c#L260 > https://elixir.bootlin.com/linux/latest/source/drivers/pci/controller/dwc/pci-meson.c#L276 > in their root port PCIe Express Device Control Register. > > Looking at the Synopsys DW-PCIe Databook, Max_Payload_Size & > Max_Read_Request_Size are used to decompose into AXI burst, but it > seems the Max_Payload_Size & Max_Read_Request_Size are set by > default to 512 but the internal Max_Payload_Size_Supported is set to > 256, thus changing these values to 256 at runtime to match and > optimize bandwidth. > > It's said, "Reducing Outbound Decomposition" : > - "Ensure that your application master does not generate bursts of > size greater than or equal to Max_Payload_Size" > > - "Program your PCIe system with a larger value of Max_Payload_Size > without exceeding Max_Payload_Size_Supported" > > - "Program your PCIe system with a larger value of Max_Read_Request > without exceeding Max_Payload_Size_Supported: > > So leaving 512 in Max_Payload_Size & Max_Read_Request leads to > Outbound Decomposition which decreases PCIe link and degrades the > AXI bus by doubling the bursts, leading to this fix to avoid > overflowing the AXI bus. > > So it seems to be still needed, I assume this *should* be handled in > the core somehow to propagate these settings to child endpoints to > match the root port Max_Payload_Size & Max_Read_Request sizes. > > Maybe by adding a core function to set these values instead of using > the dw_pcie_find_capability() & dw_pcie_write/readl_dbi() helpers > and set a state on the root port to propagate the value ? I don't have the Synopsys DW-PCIe Databook, so I'm lacking any context. The above *seems* to say that MPS/MRRS settings affect AXI bus usage. The MPS and MRRS registers are defined to affect traffic on *PCIe*. If a platform uses MPS and MRRS values to optimize transfers on non-PCIe links, that's a problem because the PCI core code that manages MPS and MRRS has no knowledge of those non-PCIe parts of the system. You might be able to deal with this in Synopsys-specific code somehow, but it's going to be a bit of a hassle because I don't want it to make maintenance of the generic MPS/MRRS code harder. Bjorn _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel