linux-kernel - [PATCH RFT 1/7] media: hantro: jpeg: Relax register writes before write starting hardware

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Date:   Fri, 24 Dec 2021 16:42:42 +0800
From:   Chen-Yu Tsai <wenst@...omium.org>
To:     Ezequiel Garcia <ezequiel@...guardiasur.com.ar>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Tomasz Figa <tfiga@...omium.org>,
        Chen-Yu Tsai <wenst@...omium.org>, linux-media@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: [PATCH RFT 1/7] media: hantro: jpeg: Relax register writes before write starting hardware

In the earlier submissions of the Hantro/Rockchip JPEG encoder driver, a
wmb() was inserted before the final register write that starts the
encoder. In v11, it was removed and the second-to-last register write
was changed to a non-relaxed write, which has an implicit wmb() [1].
The rockchip_vpu2 (then rk3399_vpu) variant is even weirder as there
is another writel_relaxed() following the non-relaxed one.

Turns out only the last writel() needs to be non-relaxed. Device I/O
mappings already guarantee strict ordering to the same endpoint, and
the writel() triggering the hardware would force all writes to memory
to be observed before the writel() to the hardware is observed.

[1] https://lore.kernel.org/linux-media/CAAFQd5ArFG0hU6MgcyLd+_UOP3+T_U-aw2FXv6sE7fGqVCVGqw@mail.gmail.com/

Signed-off-by: Chen-Yu Tsai <wenst@...omium.org>
---
 drivers/staging/media/hantro/hantro_h1_jpeg_enc.c        | 3 +--
 drivers/staging/media/hantro/rockchip_vpu2_hw_jpeg_enc.c | 3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/staging/media/hantro/hantro_h1_jpeg_enc.c b/drivers/staging/media/hantro/hantro_h1_jpeg_enc.c
index 1450013d3685..03db1c3444f8 100644
--- a/drivers/staging/media/hantro/hantro_h1_jpeg_enc.c
+++ b/drivers/staging/media/hantro/hantro_h1_jpeg_enc.c
@@ -123,8 +123,7 @@ int hantro_h1_jpeg_enc_run(struct hantro_ctx *ctx)
 		| H1_REG_AXI_CTRL_INPUT_SWAP32
 		| H1_REG_AXI_CTRL_OUTPUT_SWAP8
 		| H1_REG_AXI_CTRL_INPUT_SWAP8;
-	/* Make sure that all registers are written at this point. */
-	vepu_write(vpu, reg, H1_REG_AXI_CTRL);
+	vepu_write_relaxed(vpu, reg, H1_REG_AXI_CTRL);
 
 	reg = H1_REG_ENC_CTRL_WIDTH(MB_WIDTH(ctx->src_fmt.width))
 		| H1_REG_ENC_CTRL_HEIGHT(MB_HEIGHT(ctx->src_fmt.height))
diff --git a/drivers/staging/media/hantro/rockchip_vpu2_hw_jpeg_enc.c b/drivers/staging/media/hantro/rockchip_vpu2_hw_jpeg_enc.c
index 4df16f59fb97..b931fc5fa1a9 100644
--- a/drivers/staging/media/hantro/rockchip_vpu2_hw_jpeg_enc.c
+++ b/drivers/staging/media/hantro/rockchip_vpu2_hw_jpeg_enc.c
@@ -152,8 +152,7 @@ int rockchip_vpu2_jpeg_enc_run(struct hantro_ctx *ctx)
 		| VEPU_REG_INPUT_SWAP8
 		| VEPU_REG_INPUT_SWAP16
 		| VEPU_REG_INPUT_SWAP32;
-	/* Make sure that all registers are written at this point. */
-	vepu_write(vpu, reg, VEPU_REG_DATA_ENDIAN);
+	vepu_write_relaxed(vpu, reg, VEPU_REG_DATA_ENDIAN);
 
 	reg = VEPU_REG_AXI_CTRL_BURST_LEN(16);
 	vepu_write_relaxed(vpu, reg, VEPU_REG_AXI_CTRL);
-- 
2.34.1.448.ga2b2bfdf31-goog