Re: [PATCH v13 07/12] drm: bridge/dw_hdmi: add support for multi-byte register width access

From: Andy Yan
Date: Mon Dec 01 2014 - 07:46:51 EST

Hi Philipp:
On 2014å12æ01æ 20:04, Philipp Zabel wrote:
Am Freitag, den 28.11.2014, 17:43 +0800 schrieb Andy Yan:
Hi Zabel:
On 2014å11æ27æ 00:34, Philipp Zabel wrote:
Am Mittwoch, den 26.11.2014, 21:32 +0800 schrieb Andy Yan:
On rockchip rk3288, only word(32-bit) accesses are
permitted for hdmi registers. Byte width accesses (writeb,
readb) generate an imprecise external abort.

Signed-off-by: Andy Yan <andy.yan@xxxxxxxxxxxxxx>


Changes in v13: None
Changes in v12: None
Changes in v11: None
Changes in v10: None
Changes in v9: None
Changes in v8: None
Changes in v7: None
Changes in v6:
- refactor register access without reg_shift

Changes in v5:
- refactor reg-io-width

Changes in v4: None
Changes in v3:
- split multi-register access to one indepent patch

drivers/gpu/drm/bridge/dw_hdmi.c | 57 +++++++++++++++++++++++++++++++++++-----
1 file changed, 51 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/bridge/dw_hdmi.c b/drivers/gpu/drm/bridge/dw_hdmi.c
index a53bf63..5e88c8d 100644
--- a/drivers/gpu/drm/bridge/dw_hdmi.c
+++ b/drivers/gpu/drm/bridge/dw_hdmi.c
@@ -100,6 +100,11 @@ struct hdmi_data_info {
struct hdmi_vmode video_mode;
+union dw_reg_ptr {
+ u32 __iomem *p32;
+ u8 __iomem *p8;
I see no need to introduce this. Just explicitly multiply the offset in

Is there any disadvantage to do like this?
The compiler can help us do the explicitly multiply by this way.
Four additional lines, a new defined type, a few more changes to struct
dw_hdmi and dw_hdmi_bind necessary.

Technically I see no problem to let the compiler do the multiplication,
my issue is that it ever so slightly obfuscates the code. Instead of
just writing "* 4" in two functions, we get a new union that you need to
know about when looking at struct dw_hdmi and dw_hdmi_bind, regs.p8 is
used but never assigned directly, it's just a tiny bit of additional
effort needed to understand the code. But when the cost to avoid that is
so small...


What you said is right, I will change it in PATCH V15
thanks .
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