drivers: pcie-brcmstb: add best-effort workaround for QoS bug on bcm2712 #6183
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If a set of read requests are issued by an endpoint, they are streamed into a resynchronisation FIFO prior to exiting the RC. This FIFO has an edge case where it can drop QoS for a request to 0 if there's a single outstanding read request in the FIFO, and another is pushed when the FIFO is popped. Requests with a QoS of 0 can take hundreds of microseconds to complete. By adding an experimentally-determined amount of backpressure on the pop side, the critical level transition can largely be avoided. Signed-off-by: Jonathan Bell <[email protected]>
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See: raspberrypi/linux#6180 kernel: drivers: staging: bcm2835-isp: Respect caller's stride value See: raspberrypi/linux#5960 kernel: drivers: pcie-brcmstb: add best-effort workaround for QoS bug on bcm2712 See: raspberrypi/linux#6183 kernel: ARM: dts: Fix camera sync parameters See: raspberrypi/linux#6184 kernel: Add association between DRM panels and backlight where defined See: raspberrypi/linux#6177
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See: raspberrypi/linux#6180 kernel: drivers: staging: bcm2835-isp: Respect caller's stride value See: raspberrypi/linux#5960 kernel: drivers: pcie-brcmstb: add best-effort workaround for QoS bug on bcm2712 See: raspberrypi/linux#6183 kernel: ARM: dts: Fix camera sync parameters See: raspberrypi/linux#6184 kernel: Add association between DRM panels and backlight where defined See: raspberrypi/linux#6177
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In an attempt to chase down large request latencies on the pcie1 RC, this seems to be the only knob that gives a measureable positive effect (and at least has no observable negative effects).
It's a hack - in a heavily congested system the limit has no effect, and if an EP chooses to pipeline fewer requests then there's not much that can be done without drip-feeding transfers into the SDC, which will prevent opportunistic reordering or merging.