[xen master] xen/arm: vpl011: Handle correctly TXFE when backend in Xen

[patchbot@xxxxxxx]

commit d3784f16bbfeabde92b55ee6d5d66dcb1d82d060
Author:     Michal Orzel <michal.orzel@xxxxxxx>
AuthorDate: Wed Apr 5 13:17:49 2023 +0200
Commit:     Stefano Stabellini <stefano.stabellini@xxxxxxx>
CommitDate: Mon Apr 17 13:01:05 2023 -0700

    xen/arm: vpl011: Handle correctly TXFE when backend in Xen
    
    When backend is in Xen, the handling of data written to DR register is a
    bit special because we want to tell guest that we are always ready for new
    data to be written (i.e. no real FIFO, TXFF/BUSY never set and TXI always
    set). This conflicts with the current handling of TXFE bit, which we
    always clear and never set on a write path (we happen to set it when we
    receive char from serial input due to use of vpl011_data_avail() but this
    might never be called). This can lead to issues if a guest driver makes
    use of TXFE bit to check for TX transmission completion (such guest could
    then wait endlessly). Fix it by keeping TXFE always set to match the
    current emulation logic.
    
    Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
    Reviewed-by: Henry Wang <Henry.Wang@xxxxxxx>
    Tested-by: Henry Wang <Henry.Wang@xxxxxxx>
---
 xen/arch/arm/vpl011.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/vpl011.c b/xen/arch/arm/vpl011.c
index 0186d8a318..ff06deeb64 100644
--- a/xen/arch/arm/vpl011.c
+++ b/xen/arch/arm/vpl011.c
@@ -112,8 +112,14 @@ static void vpl011_write_data_xen(struct domain *d, 
uint8_t data)
         }
     }
 
+    /*
+     * When backend is in Xen, we tell guest we are always ready for new data
+     * to be written. This is fulfilled by having:
+     * - TXI/TXFE -> always set,
+     * - TXFF/BUSY -> never set.
+     */
     vpl011->uartris |= TXI;
-    vpl011->uartfr &= ~TXFE;
+    vpl011->uartfr |= TXFE;
     vpl011_update_interrupt_status(d);
 
     VPL011_UNLOCK(d, flags);
--
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