Re: [PATCH 1/4] media: rkvdec: Introduce a global bitwriter helper

[Nicolas Dufresne <nicolas.dufresne@collabora.com>]

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Le vendredi 27 mars 2026 à 11:16 -0400, Detlev Casanova a écrit :
> The use of structures with bitfields is good when the values are
> somewhat aligned.
> More mis-alignement means that compilers need to do more gymanstics
> to edit the fields values.
> 
> Some cases have been reported with CLang on specific architectures
> like armhf and hexagon, where the compiler would allocate a bigger
> local stack than needed or even completely freeze during compilation.
> 
> Some fixes have been provided to ease the issues, but the real fix
> here is to use a bitwriter instead of heavily unaligned bitfields.
> 
> This is a preparation commit to provide a global bitwriter interface
> for the whole driver.
> 
> Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
> ---
>  drivers/media/platform/rockchip/rkvdec/Makefile    |  1 +
>  .../platform/rockchip/rkvdec/rkvdec-bitwriter.c    | 30 ++++++++++++++++++++++
>  .../platform/rockchip/rkvdec/rkvdec-bitwriter.h    | 25 ++++++++++++++++++
>  3 files changed, 56 insertions(+)
> 
> diff --git a/drivers/media/platform/rockchip/rkvdec/Makefile b/drivers/media/platform/rockchip/rkvdec/Makefile
> index e629d571e4d8..11e2122bcbbf 100644
> --- a/drivers/media/platform/rockchip/rkvdec/Makefile
> +++ b/drivers/media/platform/rockchip/rkvdec/Makefile
> @@ -2,6 +2,7 @@ obj-$(CONFIG_VIDEO_ROCKCHIP_VDEC) += rockchip-vdec.o
>  
>  rockchip-vdec-y += \
>  		   rkvdec.o \
> +		   rkvdec-bitwriter.o \

Its just one function, with 10 lines of code, can we inline it in the header and
drop the object ?

>  		   rkvdec-cabac.o \
>  		   rkvdec-h264.o \
>  		   rkvdec-h264-common.o \
> diff --git a/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.c b/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.c
> new file mode 100644
> index 000000000000..673ebb89002b
> --- /dev/null
> +++ b/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.c
> @@ -0,0 +1,30 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Rockchip Video Decoder bit writer
> + *
> + * Copyright (C) 2026 Collabora, Ltd.
> + *      Detlev Casanova <detlev.casanova@collabora.com>
> + * Copyright (C) 2019 Collabora, Ltd.
> + *	Boris Brezillon <boris.brezillon@collabora.com>
> + */
> +
> +#include <linux/types.h>
> +#include <linux/bits.h>
> +
> +#include "rkvdec-bitwriter.h"
> +
> +void rkvdec_set_bw_field(u32 *buf, struct rkvdec_bw_field field, u32 value)
> +{
> +	u8 bit = field.offset % 32;
> +	u16 word = field.offset / 32;
> +	u64 mask = GENMASK_ULL(bit + field.len - 1, bit);
> +	u64 val = ((u64)value << bit) & mask;
> +
> +	buf[word] &= ~mask;
> +	buf[word] |= val;
> +	if (bit + field.len > 32) {
> +		buf[word + 1] &= ~(mask >> 32);
> +		buf[word + 1] |= val >> 32;
> +	}
> +}
> +
> diff --git a/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.h b/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.h
> new file mode 100644
> index 000000000000..44154f1ebc65
> --- /dev/null
> +++ b/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.h
> @@ -0,0 +1,25 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Rockchip Video Decoder bit writer
> + *
> + * Copyright (C) 2026 Collabora, Ltd.
> + *      Detlev Casanova <detlev.casanova@collabora.com>
> + * Copyright (C) 2019 Collabora, Ltd.
> + *	Boris Brezillon <boris.brezillon@collabora.com>
> + */
> +
> +#ifndef RKVDEC_BIT_WRITER_H_
> +#define RKVDEC_BIT_WRITER_H_
> +
> +#include <linux/types.h>
> +
> +struct rkvdec_bw_field {
> +	u16 offset;
> +	u8 len;
> +};
> +
> +#define BW_FIELD(_offset, _len) ((struct rkvdec_bw_field){ _offset, _len })
> +
> +void rkvdec_set_bw_field(u32 *buf, struct rkvdec_bw_field field, u32 value);
> +
> +#endif /* RKVDEC_BIT_WRITER_H_ */

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