dp8393x: don't force 32-bit register access

Commit 3fe9a838ec "dp8393x: Always use 32-bit accesses" set .impl.min_access_size
and .impl.max_access_size to 4 to try and fix the Linux jazzsonic driver which uses
32-bit accesses.

The problem with forcing the register access to 32-bit in this way is that since the
dp8393x uses 16-bit registers, a manual endian swap is required for devices on big
endian machines with 32-bit accesses.

For both access sizes and machine endians the QEMU memory API can do the right thing
automatically: all that is needed is to set .impl.min_access_size to 2 to declare that
the dp8393x implements 16-bit registers.

Normally .impl.max_access_size should also be set to 2, however that doesn't quite
work in this case since the register stride is specified using a (dynamic) it_shift
property which is applied during the MMIO access itself. The effect of this is that
for a 32-bit access the memory API performs 2 x 16-bit accesses, but the use of
it_shift within the MMIO access itself causes the register value to be repeated in both
the top 16-bits and bottom 16-bits. The Linux jazzsonic driver expects the stride to be
zero-extended up to access size and therefore fails to correctly detect the dp8393x
device due to the extra data in the top 16-bits.

The solution here is to remove .impl.max_access_size so that the memory API will
correctly zero-extend the 16-bit registers to the access size up to and including
it_shift. Since it_shift is never greater than 2 than this will always do the right
thing for both 16-bit and 32-bit accesses regardless of the machine endian, allowing
the manual endian swap code to be removed.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Fixes: 3fe9a838ec ("dp8393x: Always use 32-bit accesses")
Message-Id: <20210705214929.17222-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Finn Thain <fthain@linux-m68k.org>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

hw/net/dp8393x.c

@@ -588,15 +588,14 @@ static uint64_t dp8393x_read(void *opaque, hwaddr addr, unsigned int size)
 
     trace_dp8393x_read(reg, reg_names[reg], val, size);
 
-    return s->big_endian ? val << 16 : val;
+    return val;
 }
 
-static void dp8393x_write(void *opaque, hwaddr addr, uint64_t data,
+static void dp8393x_write(void *opaque, hwaddr addr, uint64_t val,
                           unsigned int size)
 {
     dp8393xState *s = opaque;
     int reg = addr >> s->it_shift;
-    uint32_t val = s->big_endian ? data >> 16 : data;
 
     trace_dp8393x_write(reg, reg_names[reg], val, size);
 
@@ -677,11 +676,16 @@ static void dp8393x_write(void *opaque, hwaddr addr, uint64_t data,
     }
 }
 
+/*
+ * Since .impl.max_access_size is effectively controlled by the it_shift
+ * property, leave it unspecified for now to allow the memory API to
+ * correctly zero extend the 16-bit register values to the access size up to and
+ * including it_shift.
+ */
 static const MemoryRegionOps dp8393x_ops = {
     .read = dp8393x_read,
     .write = dp8393x_write,
-    .impl.min_access_size = 4,
-    .impl.max_access_size = 4,
+    .impl.min_access_size = 2,
     .endianness = DEVICE_NATIVE_ENDIAN,
 };