soc: qcom: geni: Optimize/comment select fifo/dma mode

The functions geni_se_select_fifo_mode() and
geni_se_select_fifo_mode() are a little funny.  They read/write a
bunch of memory mapped registers even if they don't change or aren't
relevant for the current protocol.  Let's make them a little more
sane.  We'll also add a comment explaining why we don't do some of the
operations for UART.

NOTE: there is no evidence at all that this makes any performance
difference and it fixes no bugs.  However, it seems (to me) like it
makes the functions a little easier to understand.  Decreasing the
amount of times we read/write memory mapped registers is also nice,
even if we are using "relaxed" variants.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20201013142448.v2.3.I646736d3969dc47de8daceb379c6ba85993de9f4@changeidSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/soc/qcom/qcom-geni-se.c

@@ -266,49 +266,63 @@ EXPORT_SYMBOL(geni_se_init);
 static void geni_se_select_fifo_mode(struct geni_se *se)
 {
 	u32 proto = geni_se_read_proto(se);
-	u32 val;
+	u32 val, val_old;
 
 	geni_se_irq_clear(se);
 
-	val = readl_relaxed(se->base + SE_GENI_M_IRQ_EN);
+	/*
+	 * The RX path for the UART is asynchronous and so needs more
+	 * complex logic for enabling / disabling its interrupts.
+	 *
+	 * Specific notes:
+	 * - The done and TX-related interrupts are managed manually.
+	 * - We don't RX from the main sequencer (we use the secondary) so
+	 *   we don't need the RX-related interrupts enabled in the main
+	 *   sequencer for UART.
+	 */
 	if (proto != GENI_SE_UART) {
+		val_old = val = readl_relaxed(se->base + SE_GENI_M_IRQ_EN);
 		val |= M_CMD_DONE_EN | M_TX_FIFO_WATERMARK_EN;
 		val |= M_RX_FIFO_WATERMARK_EN | M_RX_FIFO_LAST_EN;
-	}
-	writel_relaxed(val, se->base + SE_GENI_M_IRQ_EN);
+		if (val != val_old)
+			writel_relaxed(val, se->base + SE_GENI_M_IRQ_EN);
 
-	val = readl_relaxed(se->base + SE_GENI_S_IRQ_EN);
-	if (proto != GENI_SE_UART)
+		val_old = val = readl_relaxed(se->base + SE_GENI_S_IRQ_EN);
 		val |= S_CMD_DONE_EN;
-	writel_relaxed(val, se->base + SE_GENI_S_IRQ_EN);
+		if (val != val_old)
+			writel_relaxed(val, se->base + SE_GENI_S_IRQ_EN);
+	}
 
-	val = readl_relaxed(se->base + SE_GENI_DMA_MODE_EN);
+	val_old = val = readl_relaxed(se->base + SE_GENI_DMA_MODE_EN);
 	val &= ~GENI_DMA_MODE_EN;
-	writel_relaxed(val, se->base + SE_GENI_DMA_MODE_EN);
+	if (val != val_old)
+		writel_relaxed(val, se->base + SE_GENI_DMA_MODE_EN);
 }
 
 static void geni_se_select_dma_mode(struct geni_se *se)
 {
 	u32 proto = geni_se_read_proto(se);
-	u32 val;
+	u32 val, val_old;
 
 	geni_se_irq_clear(se);
 
-	val = readl_relaxed(se->base + SE_GENI_M_IRQ_EN);
 	if (proto != GENI_SE_UART) {
+		val_old = val = readl_relaxed(se->base + SE_GENI_M_IRQ_EN);
 		val &= ~(M_CMD_DONE_EN | M_TX_FIFO_WATERMARK_EN);
 		val &= ~(M_RX_FIFO_WATERMARK_EN | M_RX_FIFO_LAST_EN);
-	}
-	writel_relaxed(val, se->base + SE_GENI_M_IRQ_EN);
+		if (val != val_old)
+			writel_relaxed(val, se->base + SE_GENI_M_IRQ_EN);
 
-	val = readl_relaxed(se->base + SE_GENI_S_IRQ_EN);
-	if (proto != GENI_SE_UART)
+		val_old = val = readl_relaxed(se->base + SE_GENI_S_IRQ_EN);
 		val &= ~S_CMD_DONE_EN;
-	writel_relaxed(val, se->base + SE_GENI_S_IRQ_EN);
+		if (val != val_old)
+			writel_relaxed(val, se->base + SE_GENI_S_IRQ_EN);
+	}
 
-	val = readl_relaxed(se->base + SE_GENI_DMA_MODE_EN);
+	val_old = val = readl_relaxed(se->base + SE_GENI_DMA_MODE_EN);
 	val |= GENI_DMA_MODE_EN;
-	writel_relaxed(val, se->base + SE_GENI_DMA_MODE_EN);
+	if (val != val_old)
+		writel_relaxed(val, se->base + SE_GENI_DMA_MODE_EN);
 }
 
 /**