powerpc/powernv: Properly fix LPC debugfs endianness

Endian is hard, especially when I designed a stupid FW interface, and
I should know better... oh well, this is attempt #2 at fixing this
properly. This time it seems to work with all access sizes and I
can run my flashing tool (which exercises all sort of access sizes
and types to access the SPI controller in the BMC) just fine.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/platforms/powernv/opal-lpc.c b/arch/powerpc/platforms/powernv/opal-lpc.c
index ad4b31d..e4169d6 100644 (file)
--- a/arch/powerpc/platforms/powernv/opal-lpc.c
+++ b/arch/powerpc/platforms/powernv/opal-lpc.c
@@ -216,14 +216,54 @@ static ssize_t lpc_debug_read(struct file *filp, char __user *ubuf,
                                   &data, len);
                if (rc)
                        return -ENXIO;
+
+               /*
+                * Now there is some trickery with the data returned by OPAL
+                * as it's the desired data right justified in a 32-bit BE
+                * word.
+                *
+                * This is a very bad interface and I'm to blame for it :-(
+                *
+                * So we can't just apply a 32-bit swap to what comes from OPAL,
+                * because user space expects the *bytes* to be in their proper
+                * respective positions (ie, LPC position).
+                *
+                * So what we really want to do here is to shift data right
+                * appropriately on a LE kernel.
+                *
+                * IE. If the LPC transaction has bytes B0, B1, B2 and B3 in that
+                * order, we have in memory written to by OPAL at the "data"
+                * pointer:
+                *
+                *               Bytes:      OPAL "data"   LE "data"
+                *   32-bit:   B0 B1 B2 B3   B0B1B2B3      B3B2B1B0
+                *   16-bit:   B0 B1         0000B0B1      B1B00000
+                *    8-bit:   B0            000000B0      B0000000
+                *
+                * So a BE kernel will have the leftmost of the above in the MSB
+                * and rightmost in the LSB and can just then "cast" the u32 "data"
+                * down to the appropriate quantity and write it.
+                *
+                * However, an LE kernel can't. It doesn't need to swap because a
+                * load from data followed by a store to user are going to preserve
+                * the byte ordering which is the wire byte order which is what the
+                * user wants, but in order to "crop" to the right size, we need to
+                * shift right first.
+                */
                switch(len) {
                case 4:
                        rc = __put_user((u32)data, (u32 __user *)ubuf);
                        break;
                case 2:
+#ifdef __LITTLE_ENDIAN__
+                       data >>= 16;
+#endif
                        rc = __put_user((u16)data, (u16 __user *)ubuf);
                        break;
                default:
+#ifdef __LITTLE_ENDIAN__
+                       data >>= 24;
+#endif
                        rc = __put_user((u8)data, (u8 __user *)ubuf);
                        break;
                }
@@ -263,12 +303,31 @@ static ssize_t lpc_debug_write(struct file *filp, const char __user *ubuf,
                        else if (todo > 1 && (pos & 1) == 0)
                                len = 2;
                }
+
+               /*
+                * Similarly to the read case, we have some trickery here but
+                * it's different to handle. We need to pass the value to OPAL in
+                * a register whose layout depends on the access size. We want
+                * to reproduce the memory layout of the user, however we aren't
+                * doing a load from user and a store to another memory location
+                * which would achieve that. Here we pass the value to OPAL via
+                * a register which is expected to contain the "BE" interpretation
+                * of the byte sequence. IE: for a 32-bit access, byte 0 should be
+                * in the MSB. So here we *do* need to byteswap on LE.
+                *
+                *           User bytes:    LE "data"  OPAL "data"
+                *  32-bit:  B0 B1 B2 B3    B3B2B1B0   B0B1B2B3
+                *  16-bit:  B0 B1          0000B1B0   0000B0B1
+                *   8-bit:  B0             000000B0   000000B0
+                */
                switch(len) {
                case 4:
                        rc = __get_user(data, (u32 __user *)ubuf);
+                       data = cpu_to_be32(data);
                        break;
                case 2:
                        rc = __get_user(data, (u16 __user *)ubuf);
+                       data = cpu_to_be16(data);
                        break;
                default:
                        rc = __get_user(data, (u8 __user *)ubuf);