Re: [PATCH v4 01/16] drm/dsi: Add message to packet translator

[Andrzej Hajda <a.hajda@samsung.com>]

On 11/04/2014 03:39 PM, Thierry Reding wrote:
> On Tue, Nov 04, 2014 at 03:21:14PM +0100, Andrzej Hajda wrote:
>> On 11/04/2014 02:58 PM, Thierry Reding wrote:
>>> On Tue, Nov 04, 2014 at 12:43:21PM +0100, Andrzej Hajda wrote:
>>>> On 11/03/2014 10:13 AM, Thierry Reding wrote:
> [...]
>>>>> diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
> [...]
>>>>> +		packet->header[1] = (msg->tx_len >> 0) & 0xff;
>>>>> +		packet->header[2] = (msg->tx_len >> 8) & 0xff;
>>>>> +
>>>>> +		packet->payload_length = msg->tx_len;
>>>>> +		packet->payload = tx;
>>>>> +	} else {
>>>>> +		packet->header[1] = (msg->tx_len > 0) ? tx[0] : 0;
>>>>> +		packet->header[2] = (msg->tx_len > 1) ? tx[1] : 0;
>>>>> +	}
>>>>> +
>>>>> +	packet->size = sizeof(packet->header) + packet->payload_length;
>>>> size seems to be completely useless.
>>> It's not. Tegra has two FIFOs that can be selected depending on the size
>>> of a transfer. I use this field to detect which FIFO needs to be
>>> selected.
>> But size is always equal payload_length + 4, so it does not carry any
>> additional information.
> Right, but it's out of convenience to prevent every driver from doing
> this again. It's part of the help that the helper provides. =)
>
>>>>> +
>>>>> +	return 0;
>>>>> +}
>>>>> +EXPORT_SYMBOL(mipi_dsi_create_packet);
>>>>> +
>>>>> +/**
>>>>>   * mipi_dsi_dcs_write - send DCS write command
>>>>>   * @dsi: DSI device
>>>>>   * @data: pointer to the command followed by parameters
>>>>> diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
>>>>> index 8569dc5a1026..663aa68826f4 100644
>>>>> --- a/include/drm/drm_mipi_dsi.h
>>>>> +++ b/include/drm/drm_mipi_dsi.h
>>>>> @@ -44,6 +44,24 @@ struct mipi_dsi_msg {
>>>>>  };
>>>>>  
>>>>>  /**
>>>>> + * struct mipi_dsi_packet - represents a MIPI DSI packet in protocol format
>>>>> + * @size: size (in bytes) of the packet
>>>>> + * @header: the four bytes that make up the header (Data ID, Word Count or
>>>>> + *     Packet Data, and ECC)
>>>>> + * @payload_length: number of bytes in the payload
>>>>> + * @payload: a pointer to a buffer containing the payload, if any
>>>>> + */
>>>>> +struct mipi_dsi_packet {
>>>>> +	size_t size;
>>>>> +	u8 header[4];
>>>> I wonder if it wouldn't be good to make it u32 or at least anonymous union:
>>>> union {
>>>>     u8 header[4];
>>>>     u32 header32;
>>>> };
>>> I'm not sure this is very useful. It's pretty trivial how you
>>> concatenate the individual bytes and it actually remove any ambiguity
>>> about the endianness.
>> This looks better:
>>
>> val = le16_to_cpu(pkt->header32);
>> writel(val, REG);
>>
>> than this:
>>
>> val = pkt->header[2] << 16 | pkt->header[1] << 8 | pkt->header[0];
>> writel(val, REG);
> I disagree. =) Having the individual bytes makes their ordering very
> explicit.
>

Wow, you want to keep size field to prevent drivers from adding
sometimes 4 to payload,
but you do not want to simplify header calculation that is much more
complicated :)

Regards
Andrzej




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