linux-kernel - Re: [PATCH 1/3] drm/dsi: Add support for Turn on/Shutdown peripheral packets

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Message-ID: <20151124093356.GA6149@ulmo>
Date:	Tue, 24 Nov 2015 10:33:56 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	bjorn@...o.se
Cc:	David Airlie <airlied@...ux.ie>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 1/3] drm/dsi: Add support for Turn on/Shutdown peripheral
 packets

On Fri, Oct 30, 2015 at 05:38:26PM -0700, bjorn@...o.se wrote:
> From: Werner Johansson <werner.johansson@...ymobile.com>
> 
> The MIPI_DSI_TURN_ON_PERIPHERAL and MIPI_DSI_SHUTDOWN_PERIPHERAL
> packets are required for some panels, one example being the
> Panasonic VVX10F034N00 panel.
> 
> Signed-off-by: Werner Johansson <werner.johansson@...ymobile.com>
> Signed-off-by: Bjorn Andersson <bjorn.andersson@...ymobile.com>
> ---
>  drivers/gpu/drm/drm_mipi_dsi.c | 47 ++++++++++++++++++++++++++++++++++++++++++
>  include/drm/drm_mipi_dsi.h     |  3 +++
>  2 files changed, 50 insertions(+)

This being a dependency on 3/3 it would've been good to send this To: me
as well. I hadn't seen this in my inbox and had simply discarded it as
being independent. Of course if I had properly checked the build I
would've noticed...

Anyway, I've applied this with slight modifications. I removed the raw
short write helper. I don't think it buys us much and we already have an
open-coded variant in mipi_dsi_set_maximum_return_packet_size(). I've
also moved these functions closer to the Set Maximum Return Packet Size
command implementation because they are part of the same command set. I
also made the functions return int instead of ssize_t to avoid potential
confusion about their return value (ssize_t implies "number of bytes
transferred, or a negative error code on failure).

One minor comment below...

> diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
> index 2d5ca8ee..13b4a9c 100644
> --- a/drivers/gpu/drm/drm_mipi_dsi.c
> +++ b/drivers/gpu/drm/drm_mipi_dsi.c
> @@ -862,6 +862,53 @@ int mipi_dsi_dcs_set_pixel_format(struct mipi_dsi_device *dsi, u8 format)
>  }
>  EXPORT_SYMBOL(mipi_dsi_dcs_set_pixel_format);
>  
> +/**
> + * mipi_dsi_raw_short_write() - Sends a data-less short DSI packet
> + * @dsi: DSI peripheral device
> + * @type: Data Type of packet to send
> + *
> + * Return: 0 on success or a negative error code on failure.
> + */
> +static ssize_t mipi_dsi_raw_short_write(struct mipi_dsi_device *dsi, u8 type)
> +{
> +	u8 dummy[2] = { 0, 0 };
> +	struct mipi_dsi_msg msg = {
> +		.channel = dsi->channel,
> +		.tx_buf = dummy,
> +		.tx_len = sizeof(dummy),
> +		.type = type
> +	};
> +
> +	if (mipi_dsi_packet_format_is_short(type))
> +		return mipi_dsi_device_transfer(dsi, &msg);
> +	else
> +		return -1;
> +}

When the kerneldoc comment says "negative error code", developers will
expect one of the -E* constants. -1 maps to -EPERM (operation not
permitted), which isn't a good error code for this case. This is also an
internal function, so these errors really should never happen.

Anyway, since I removed this helper this isn't an issue anymore, just
wanted to mention it.

Thanks,
Thierry

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