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On BCM2835, there is no hardware guarantee that multiple outstanding reads to different peripherals will complete in-order. The FIQ code uses peripheral reads without barriers for performance, so in the case where a read to a slow peripheral was issued immediately prior to FIQ entry, the first peripheral read that the FIQ did could end up with wrong read data returned. Add dsb(sy) on entry so that all outstanding reads are retired. The FIQ only issues reads to the dwc_otg core, so per-read barriers in the handler itself are not required. On BCM2836 and BCM2837 the barrier is not strictly required due to differences in how the peripheral bus is implemented, but having arch-specific handlers that introduce different latencies is risky. Signed-off-by: Jonathan Bell <jonathan@raspberrypi.org>