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Date: Tue, 27 Mar 2018 12:57:54 +0300
From: Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
To: Andrzej Hajda <a.hajda@...sung.com>,
Jacopo Mondi <jacopo+renesas@...ndi.org>,
<architt@...eaurora.org>, <Laurent.pinchart@...asonboard.com>,
<airlied@...ux.ie>, <horms@...ge.net.au>, <magnus.damm@...il.com>,
<geert@...ux-m68k.org>, <niklas.soderlund@...natech.se>,
<sergei.shtylyov@...entembedded.com>, <robh+dt@...nel.org>,
<mark.rutland@....com>
CC: <dri-devel@...ts.freedesktop.org>,
<linux-renesas-soc@...r.kernel.org>, <devicetree@...r.kernel.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/3] drm: bridge: Add thc63lvd1024 LVDS decoder driver
Hi Andrzej,
On 03/27/2018 10:28 AM, Andrzej Hajda wrote:
> On 27.03.2018 08:24, Vladimir Zapolskiy wrote:
>> Hi Jacopo,
>>
>> On 03/16/2018 05:16 PM, Jacopo Mondi wrote:
>>> Add DRM bridge driver for Thine THC63LVD1024 LVDS to digital parallel
>>> output converter.
>>>
>>> Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
>>> Reviewed-by: Andrzej Hajda <a.hajda@...sung.com>
>>> Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
>>> ---
>>> drivers/gpu/drm/bridge/Kconfig | 6 +
>>> drivers/gpu/drm/bridge/Makefile | 1 +
>>> drivers/gpu/drm/bridge/thc63lvd1024.c | 255 ++++++++++++++++++++++++++++++++++
>>> 3 files changed, 262 insertions(+)
>>> create mode 100644 drivers/gpu/drm/bridge/thc63lvd1024.c
>>>
>>> diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
>>> index 3b99d5a..5815a20 100644
>>> --- a/drivers/gpu/drm/bridge/Kconfig
>>> +++ b/drivers/gpu/drm/bridge/Kconfig
>>> @@ -92,6 +92,12 @@ config DRM_SII9234
>>> It is an I2C driver, that detects connection of MHL bridge
>>> and starts encapsulation of HDMI signal.
>>>
>>> +config DRM_THINE_THC63LVD1024
>>> + tristate "Thine THC63LVD1024 LVDS decoder bridge"
>>> + depends on OF
>>> + ---help---
>>> + Thine THC63LVD1024 LVDS/parallel converter driver.
>>> +
>>> config DRM_TOSHIBA_TC358767
>>> tristate "Toshiba TC358767 eDP bridge"
>>> depends on OF
>>> diff --git a/drivers/gpu/drm/bridge/Makefile b/drivers/gpu/drm/bridge/Makefile
>>> index 373eb28..fd90b16 100644
>>> --- a/drivers/gpu/drm/bridge/Makefile
>>> +++ b/drivers/gpu/drm/bridge/Makefile
>>> @@ -8,6 +8,7 @@ obj-$(CONFIG_DRM_PARADE_PS8622) += parade-ps8622.o
>>> obj-$(CONFIG_DRM_SIL_SII8620) += sil-sii8620.o
>>> obj-$(CONFIG_DRM_SII902X) += sii902x.o
>>> obj-$(CONFIG_DRM_SII9234) += sii9234.o
>>> +obj-$(CONFIG_DRM_THINE_THC63LVD1024) += thc63lvd1024.o
>>> obj-$(CONFIG_DRM_TOSHIBA_TC358767) += tc358767.o
>>> obj-$(CONFIG_DRM_ANALOGIX_DP) += analogix/
>>> obj-$(CONFIG_DRM_I2C_ADV7511) += adv7511/
>>> diff --git a/drivers/gpu/drm/bridge/thc63lvd1024.c b/drivers/gpu/drm/bridge/thc63lvd1024.c
>>> new file mode 100644
>>> index 0000000..02a54c2
>>> --- /dev/null
>>> +++ b/drivers/gpu/drm/bridge/thc63lvd1024.c
>>> @@ -0,0 +1,255 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +/*
>>> + * THC63LVD1024 LVDS to parallel data DRM bridge driver.
>>> + *
>>> + * Copyright (C) 2018 Jacopo Mondi <jacopo+renesas@...ndi.org>
>>> + */
>>> +
>>> +#include <drm/drmP.h>
>>> +#include <drm/drm_bridge.h>
>>> +#include <drm/drm_panel.h>
>>> +
>>> +#include <linux/gpio/consumer.h>
>>> +#include <linux/of_graph.h>
>>> +#include <linux/regulator/consumer.h>
>>> +
>>> +enum {
>>> + THC63_LVDS_IN0,
>>> + THC63_LVDS_IN1,
>>> + THC63_DIGITAL_OUT0,
>>> + THC63_DIGITAL_OUT1,
>>> +};
>>> +
>>> +static const char * const thc63_reg_names[] = {
>>> + "vcc", "lvcc", "pvcc", "cvcc",
>>> +};
>>> +
>>> +struct thc63_dev {
>>> + struct device *dev;
>>> +
>>> + struct regulator *vccs[ARRAY_SIZE(thc63_reg_names)];
>>> +
>>> + struct gpio_desc *pdwn;
>>> + struct gpio_desc *oe;
>>> +
>>> + struct drm_bridge bridge;
>>> + struct drm_bridge *next;
>>> +};
>>> +
>>> +static inline struct thc63_dev *to_thc63(struct drm_bridge *bridge)
>>> +{
>>> + return container_of(bridge, struct thc63_dev, bridge);
>>> +}
>>> +
>>> +static int thc63_attach(struct drm_bridge *bridge)
>>> +{
>>> + struct thc63_dev *thc63 = to_thc63(bridge);
>>> +
>>> + return drm_bridge_attach(bridge->encoder, thc63->next, bridge);
>>> +}
>>> +
>>> +static void thc63_enable(struct drm_bridge *bridge)
>>> +{
>>> + struct thc63_dev *thc63 = to_thc63(bridge);
>>> + struct regulator *vcc;
>>> + int i;
>> unsigned int i;
>
> Why? You are introducing silly bug this way, see few lines below.
You see that the comment was too terse to describe the context in full.
Thank you for exposing a flaw.
>>
>>> +
>>> + for (i = 0; i < ARRAY_SIZE(thc63->vccs); i++) {
>>> + vcc = thc63->vccs[i];
>>> + if (!vcc)
>>> + continue;
>>> +
>>> + if (regulator_enable(vcc))
>>> + goto error_vcc_enable;
>>> + }
>>> +
>>> + if (thc63->pdwn)
>>> + gpiod_set_value(thc63->pdwn, 0);
>>> +
>>> + if (thc63->oe)
>>> + gpiod_set_value(thc63->oe, 1);
>>> +
>>> + return;
>>> +
>>> +error_vcc_enable:
>>> + dev_err(thc63->dev, "Failed to enable regulator %s\n",
>>> + thc63_reg_names[i]);
>>> +
>>> + for (i = i - 1; i >= 0; i--) {
>
> Here, the loop will not end if you define i unsigned.
>
> I know one can change the loop, to make it working with unsigned. But
> this clearly shows that using unsigned is more risky.
> What are advantages of unsigned vs int in this case. Are there some
> guidelines/discussions about it?
>
The reason is pretty simple, like Geert said it might be a personal reason,
but a code pattern
int i;
...
for (i = 0; i < ARRAY_SIZE(...); i++)
is my own red rag, because I've seen the pattern so many times, and every
time here a compiler produces a W=12 (or -Wsign-compare) warning like
the next one:
drivers/gpu/drm/bridge/thc63lvd1024.c:182:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
for (i = 0; i < ARRAY_SIZE(thc63->vccs); i++, reg++) {
^
Usually a conversion from 'int i' to 'unsigned int i' is trivial, and lowering
of a noise level in compiler's output is seen as beneficial, because it gives
more chances to people to capture a real problem.
--
With best wishes,
Vladimir
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