drm/tegra: dp: Support address-only I2C-over-AUX transactions

Certain types of I2C-over-AUX transactions require that only the address
is transferred. Detect this by looking at the AUX message's size and set
the address-only bit appropriately.

Signed-off-by: Thierry Reding <treding@nvidia.com>

2 files changed, 32 insertions, 13 deletions

diff --git a/drivers/gpu/drm/tegra/dpaux.c b/drivers/gpu/drm/tegra/dpaux.c
index d536ed381fbd..005c19bd92df 100644
--- a/drivers/gpu/drm/tegra/dpaux.c
+++ b/drivers/gpu/drm/tegra/dpaux.c
@@ -99,55 +99,73 @@ static void tegra_dpaux_read_fifo(struct tegra_dpaux *dpaux, u8 *buffer,
 static ssize_t tegra_dpaux_transfer(struct drm_dp_aux *aux,
                                     struct drm_dp_aux_msg *msg)
 {
-        unsigned long value = DPAUX_DP_AUXCTL_TRANSACTREQ;
         unsigned long timeout = msecs_to_jiffies(250);
         struct tegra_dpaux *dpaux = to_dpaux(aux);
         unsigned long status;
         ssize_t ret = 0;
+        u32 value;
 
-        if (msg->size < 1 || msg->size > 16)
+        /* Tegra has 4x4 byte DP AUX transmit and receive FIFOs. */
+        if (msg->size > 16)
                 return -EINVAL;
 
-        tegra_dpaux_writel(dpaux, msg->address, DPAUX_DP_AUXADDR);
+        /*
+         * Allow zero-sized messages only for I2C, in which case they specify
+         * address-only transactions.
+         */
+        if (msg->size < 1) {
+                switch (msg->request & ~DP_AUX_I2C_MOT) {
+                case DP_AUX_I2C_WRITE:
+                case DP_AUX_I2C_READ:
+                        value = DPAUX_DP_AUXCTL_CMD_ADDRESS_ONLY;
+                        break;
+
+                default:
+                        return -EINVAL;
+                }
+        } else {
+                /* For non-zero-sized messages, set the CMDLEN field. */
+                value = DPAUX_DP_AUXCTL_CMDLEN(msg->size - 1);
+        }
 
         switch (msg->request & ~DP_AUX_I2C_MOT) {
         case DP_AUX_I2C_WRITE:
                 if (msg->request & DP_AUX_I2C_MOT)
-                        value = DPAUX_DP_AUXCTL_CMD_MOT_WR;
+                        value |= DPAUX_DP_AUXCTL_CMD_MOT_WR;
                 else
-                        value = DPAUX_DP_AUXCTL_CMD_I2C_WR;
+                        value |= DPAUX_DP_AUXCTL_CMD_I2C_WR;
 
                 break;
 
         case DP_AUX_I2C_READ:
                 if (msg->request & DP_AUX_I2C_MOT)
-                        value = DPAUX_DP_AUXCTL_CMD_MOT_RD;
+                        value |= DPAUX_DP_AUXCTL_CMD_MOT_RD;
                 else
-                        value = DPAUX_DP_AUXCTL_CMD_I2C_RD;
+                        value |= DPAUX_DP_AUXCTL_CMD_I2C_RD;
 
                 break;
 
         case DP_AUX_I2C_STATUS:
                 if (msg->request & DP_AUX_I2C_MOT)
-                        value = DPAUX_DP_AUXCTL_CMD_MOT_RQ;
+                        value |= DPAUX_DP_AUXCTL_CMD_MOT_RQ;
                 else
-                        value = DPAUX_DP_AUXCTL_CMD_I2C_RQ;
+                        value |= DPAUX_DP_AUXCTL_CMD_I2C_RQ;
 
                 break;
 
         case DP_AUX_NATIVE_WRITE:
-                value = DPAUX_DP_AUXCTL_CMD_AUX_WR;
+                value |= DPAUX_DP_AUXCTL_CMD_AUX_WR;
                 break;
 
         case DP_AUX_NATIVE_READ:
-                value = DPAUX_DP_AUXCTL_CMD_AUX_RD;
+                value |= DPAUX_DP_AUXCTL_CMD_AUX_RD;
                 break;
 
         default:
                 return -EINVAL;
         }
 
-        value |= DPAUX_DP_AUXCTL_CMDLEN(msg->size - 1);
+        tegra_dpaux_writel(dpaux, msg->address, DPAUX_DP_AUXADDR);
         tegra_dpaux_writel(dpaux, value, DPAUX_DP_AUXCTL);
 
         if ((msg->request & DP_AUX_I2C_READ) == 0) {
@@ -198,7 +216,7 @@ static ssize_t tegra_dpaux_transfer(struct drm_dp_aux *aux,
                 break;
         }
 
-        if (msg->reply == DP_AUX_NATIVE_REPLY_ACK) {
+        if ((msg->size > 0) && (msg->reply == DP_AUX_NATIVE_REPLY_ACK)) {
                 if (msg->request & DP_AUX_I2C_READ) {
                         size_t count = value & DPAUX_DP_AUXSTAT_REPLY_MASK;
 
diff --git a/drivers/gpu/drm/tegra/dpaux.h b/drivers/gpu/drm/tegra/dpaux.h
index 4f5bf10fdff9..806e245ca787 100644
--- a/drivers/gpu/drm/tegra/dpaux.h
+++ b/drivers/gpu/drm/tegra/dpaux.h
@@ -32,6 +32,7 @@
 #define DPAUX_DP_AUXCTL_CMD_I2C_RQ (2 << 12)
 #define DPAUX_DP_AUXCTL_CMD_I2C_RD (1 << 12)
 #define DPAUX_DP_AUXCTL_CMD_I2C_WR (0 << 12)
+#define DPAUX_DP_AUXCTL_CMD_ADDRESS_ONLY (1 << 8)
 #define DPAUX_DP_AUXCTL_CMDLEN(x) ((x) & 0xff)
 
 #define DPAUX_DP_AUXSTAT 0x31